  We discuss non-perturbative aspects of string effective field theories with
N=1 supersymmetry in four dimensions. By the use of a scalar potential, which
is on-shell invariant under the supersymmetric duality of the dilaton, we study
gaugino condensation in (2,2) symmetric Z_{N} orbifold compactifications. The
duality under consideration relates a two-form antisymmetric tensor to a
pseudoscalar. We show, that our approach is independent of the
superfield-representation of the dilaton and preserves the U(1)_{PQ}
Peccei-Quinn symmetry exactly.
