  We show how an F-theory compactified on a Calabi-Yau (n+1)-fold in
appropriate weak coupling limit reduces formally to an orientifold of type IIB
theory compactified on an auxiliary complex n-fold. In some cases (but not
always) if the original (n+1)-fold is singular, then the auxiliary n-fold is
also singular. We illustrate this by analysing F-theory on elliptically fibered
Calabi-Yau 3-folds on base $F_n$.
