diff --git a/code/m_parameters.f90 b/code/m_parameters.f90
index a096fa2..2e868e4 100644
--- a/code/m_parameters.f90
+++ b/code/m_parameters.f90
@@ -12,6 +12,7 @@ module m_parameters
      ! Transport Properties
      real :: vis,sc,diff
      real :: d_turb
+     integer :: d_mode
      real :: scp,prp,lep,vis0p,rod
      real :: lewis, le_a, le_x
 
@@ -122,6 +123,7 @@ contains
         CALL READ_PARAMETER (nrxn)     ! number of reactions
 
         CALL READ_PARAMETER (d_turb)   ! turbulent diffusivity
+        CALL READ_PARAMETER (d_mode)   ! diffusivity formula selector
 
         ! READ(itape,*) cdum,table_size
         ! WRITE(otape,*) cdum,table_size
diff --git a/code/sample/itape.py b/code/sample/itape.py
index ea3c0bf..1a5c078 100755
--- a/code/sample/itape.py
+++ b/code/sample/itape.py
@@ -40,6 +40,7 @@ le_x      0.20
 nsp       2
 nrxn      1
 d_turb    0
+d_mode    0
 '''
 ).split()
 
diff --git a/code/ysolve.f90 b/code/ysolve.f90
index 37d8ef9..fed339a 100644
--- a/code/ysolve.f90
+++ b/code/ysolve.f90
@@ -89,17 +89,33 @@
 !------------------------------------------------------------------------
 
      SUBROUTINE solve
-       IF (reaction_type == "onestep") THEN
-         CALL solve_ (fonestep, SET_IC_ONESTEP)
-       ELSE IF (reaction_type == "twostep") THEN
-         CALL solve_ (fns, SET_IC_TWOSTEP)
+       IF (d_mode == 0) THEN
+         CALL solve_with_diffusivity (update_dm)
+       ELSE IF (d_mode == 1) THEN
+         CALL solve_with_diffusivity (update_dm)
        ELSE
-         WRITE(*,*)'ERROR, UNSUPPORTED REACTION ', trim(reaction_type), '. STOPPING'
+         WRITE(*,*)'ERROR, UNSUPPORTED DIFFUSIVITY ', d_mode, '. STOPPING'
          STOP
        END IF
      END SUBROUTINE solve
 
-     SUBROUTINE solve_ (rhs, SET_IC)
+     SUBROUTINE solve_with_diffusivity (calc_diff)
+      INTERFACE
+       SUBROUTINE calc_diff
+       END SUBROUTINE calc_diff
+      END INTERFACE
+
+       IF (reaction_type == "onestep") THEN
+         CALL solve_ (fonestep, SET_IC_ONESTEP, calc_diff)
+       ELSE IF (reaction_type == "twostep") THEN
+         CALL solve_ (fns, SET_IC_TWOSTEP, calc_diff)
+       ELSE
+         WRITE(*,*)'ERROR, UNSUPPORTED REACTION ', trim(reaction_type), '. STOPPING'
+         STOP
+       END IF
+     END SUBROUTINE solve_with_diffusivity
+
+     SUBROUTINE solve_ (rhs, SET_IC, calc_diff)
 
       INTERFACE
        SUBROUTINE rhs(r1,f)
@@ -109,6 +125,9 @@
 
        SUBROUTINE SET_IC
        END SUBROUTINE SET_IC
+
+       SUBROUTINE calc_diff
+       END SUBROUTINE calc_diff
       END INTERFACE
 
       INTEGER :: i,j,k,savenum