diff --git a/IPhreeqc.h b/IPhreeqc.h
index 93adac90..4c04c426 100644
--- a/IPhreeqc.h
+++ b/IPhreeqc.h
@@ -1672,9 +1672,9 @@ Headings
  *  FUNCTION SetBasicFortranCallback(ID,FCN)
  *    INTEGER(KIND=4),  INTENT(IN)  :: ID
  *    INTERFACE
- *      DOUBLE PRECISION FUNCTION FCN(x1, x2, str)
- *        DOUBLE PRECISION, INTENT(in) :: x1
- *        DOUBLE PRECISION, INTENT(in) :: x2
+ *      real(kind=8) FUNCTION FCN(x1, x2, str)
+ *        real(kind=8), INTENT(in) :: x1
+ *        real(kind=8), INTENT(in) :: x2
  *        CHARACTER(*), INTENT(in)     :: str
  *      END FUNCTION
  *    END INTERFACE
diff --git a/IPhreeqc.hpp b/IPhreeqc.hpp
index 22bdf150..c79957b5 100644
--- a/IPhreeqc.hpp
+++ b/IPhreeqc.hpp
@@ -713,7 +713,7 @@ public:
 	 *  Sets a Fortran callback function for Basic programs. The syntax for the Basic command is
 	 *  10 result = CALLBACK(x1, x2, string$)
 	 *  The syntax for the Fortran function is
-	 *  double precision my_callback(x1, x2, string), where x1 and x2 are double precision and string is a character variable.
+	 *  real(kind=8) my_callback(x1, x2, string), where x1 and x2 are real(kind=8) and string is a character variable.
 	 *  @param fcn              The name of a user-defined function.
 	 *  @see                    SetBasicCallback
 	 */
diff --git a/IPhreeqc_interface.F90 b/IPhreeqc_interface.F90
index 6c726c83..45e6db0c 100644
--- a/IPhreeqc_interface.F90
+++ b/IPhreeqc_interface.F90
@@ -688,7 +688,7 @@ INTEGER FUNCTION GetSelectedOutputValue(id, row, col, vtype, dvalue, svalue, sle
     END INTERFACE
     INTEGER, INTENT(in) :: id, row, col
     INTEGER, INTENT(out) :: vtype
-    DOUBLE PRECISION, INTENT(out) :: dvalue
+    real(kind=8), INTENT(out) :: dvalue
     CHARACTER(len=*), INTENT(out) :: svalue
     INTEGER, INTENT(out), OPTIONAL :: slength
     INTEGER :: sz, sz_fortran
@@ -898,9 +898,9 @@ INTEGER FUNCTION SetBasicFortranCallback(id, fcn)
             IMPLICIT NONE
             INTEGER, INTENT(in) :: id
             INTERFACE
-                DOUBLE PRECISION FUNCTION fcn(x1, x2, str, l)
+                real(kind=8) FUNCTION fcn(x1, x2, str, l)
                     INTEGER, INTENT(in)   :: l
-                    DOUBLE PRECISION, INTENT(in) :: x1, x2
+                    real(kind=8), INTENT(in) :: x1, x2
                     CHARACTER, INTENT(in) :: str(*)
                 END FUNCTION fcn
            END INTERFACE
@@ -908,9 +908,9 @@ INTEGER FUNCTION SetBasicFortranCallback(id, fcn)
     END INTERFACE
     INTEGER, INTENT(in) :: id     
     INTERFACE
-        DOUBLE PRECISION FUNCTION fcn(x1, x2, str, l) 
+        real(kind=8) FUNCTION fcn(x1, x2, str, l) 
             INTEGER, INTENT(in)          :: l
-            DOUBLE PRECISION, INTENT(in) :: x1, x2
+            real(kind=8), INTENT(in) :: x1, x2
             CHARACTER, INTENT(in) :: str(*)
         END FUNCTION fcn
     END INTERFACE
diff --git a/phreeqcpp/cl1.cpp b/phreeqcpp/cl1.cpp
index 107a8ad9..b0dd1be4 100644
--- a/phreeqcpp/cl1.cpp
+++ b/phreeqcpp/cl1.cpp
@@ -142,7 +142,7 @@ cl1(int k, int l, int m, int n,
 /*        AT LEAST NKLMD COLUMNS USED FOR WORKSPACE. */
 /* S      INTEGER ARRAY OF SIZE AT LEAST KLMD, USED FOR */
 /*        WORKSPACE. */
-/*      DOUBLE PRECISION DBLE */
+/*      real(kind=8) DBLE */
 /*      REAL */
 
 /* INITIALIZATION. */
diff --git a/phreeqcpp/sundialsmath.h b/phreeqcpp/sundialsmath.h
index 59e49d3a..4f240e81 100644
--- a/phreeqcpp/sundialsmath.h
+++ b/phreeqcpp/sundialsmath.h
@@ -13,10 +13,10 @@
  * listed here work with the type realtype as defined in                  *
  * sundialstypes.h.                                                       *
  * To do single precision floating point arithmetic, set the type         *
- * realtype to be float. To do double precision arithmetic, set           *
+ * realtype to be float. To do real(kind=8) arithmetic, set           *
  * the type realtype to be double. The default implementations            *
  * for RPowerR and RSqrt call standard math library functions             *
- * which do double precision arithmetic. If this is unacceptable          *
+ * which do real(kind=8) arithmetic. If this is unacceptable          *
  * when realtype is float, then the user should re-implement              *
  * these two routines by calling single precision routines                *
  * available on his/her machine.                                          *
diff --git a/phreeqcpp/sundialstypes.h b/phreeqcpp/sundialstypes.h
index 1003410f..ddf6c84a 100644
--- a/phreeqcpp/sundialstypes.h
+++ b/phreeqcpp/sundialstypes.h
@@ -25,7 +25,7 @@
  * The types for realtype and integertype below have been set to          *
  * double and long int, respectively. A user should modify these          *
  * type declarations as he/she sees fit. For example, if a user           *
- * wants the work with type float because double precision                *
+ * wants the work with type float because real(kind=8)                *
  * floating point arithmetic is too expensive on the user's               *
  * machine, then the definition below should be changed to:               *
  *                                                                        *