diff --git a/sit b/sit
index 8a19b20..84ab246 100755
--- a/sit
+++ b/sit
@@ -125,13 +125,14 @@ fi
 PACKAGENAME=${PACKAGENAME:=$PACKAGE}
 
 # construct the final installation directory path
+if [ -z $PREFIX ] ; then
 PREFIX="${PREFIX_BASE}/${CATEGORY}/${PACKAGE}/${VERSION}"
 PREFIX_SUFFIX=${MPI:+"-$MPI"}${MPI_VERSION_NUM:+"-$MPI_VERSION_NUM"}
 PREFIX_SUFFIX=$PREFIX_SUFFIX${COMPILER:+"-$COMPILER"}${COMPILER_VERSION:+"-$COMPILER_VERSION"}
 PREFIX_SUFFIX=$PREFIX_SUFFIX${PACKAGE_DESCRIPTOR}
 
-
 PREFIX=$PREFIX${PREFIX_SUFFIX:+"$PREFIX_SUFFIX"}
+fi
 
 # final working directory path
 WORKDIR=${WORKDIR_BASE}/${CATEGORY}/${PACKAGE}/${VERSION}$PREFIX_SUFFIX