diff --git a/CMakeLists.txt b/CMakeLists.txt
index f3b5ce203dfdde2d3a9eccc891ece9979f6bea24..0b9aabcff5dcf9a9426197deec67fa92ce13a319 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -616,6 +616,12 @@ endif(ENABLE_TAUCS)
 if(ENABLE_PETSC)
   set(ENV_PETSC_DIR $ENV{PETSC_DIR})
   set(ENV_PETSC_ARCH $ENV{PETSC_ARCH})
+  if(NOT ENV_PETSC_DIR)
+     message("WARNING: Export PETSC_DIR before calling cmake")
+  endif(NOT ENV_PETSC_DIR)
+  if(NOT ENV_PETSC_ARCH)
+     message("WARNING: Export PETSC_ARCH before calling cmake")
+  endif(NOT ENV_PETSC_ARCH)
   if(EXISTS ${ENV_PETSC_DIR}/${ENV_PETSC_ARCH}/conf/petscvariables)
     set_config_option(HAVE_PETSC "PETSc")
     file(STRINGS ${ENV_PETSC_DIR}/${ENV_PETSC_ARCH}/conf/petscvariables 
@@ -647,6 +653,9 @@ if(ENABLE_PETSC)
     # find slepc
     if(ENABLE_SLEPC)
       set(ENV_SLEPC_DIR $ENV{SLEPC_DIR})
+      if(NOT ENV_SLEPC_DIR)
+         message("WARNING: Export SLEPC_DIR before calling cmake")
+      endif(NOT ENV_SLEPC_DIR)
       find_library(SLEPC_LIB slepc PATHS ${ENV_SLEPC_DIR}/${ENV_PETSC_ARCH}/lib)
       if(SLEPC_LIB)
         set_config_option(HAVE_SLEPC "SLEPc")