  Full one-loop electroweak corrections to the on-shell decay $H^+ \to W^+A^0$
are computed in the framework of models with two Higgs doublets (THDM). Such a
decay may be dominant for $H^{\pm}$ over a wide range of parameter space
relevant at present and future colliders. We show that the corrections may
approach 40% and in particular are sensitive to $\lambda_5$, which parametrizes
the discrete symmetry breaking term. We suggest that a measurement of the
branching ratio of $H^+ \to W^+A^0$ may offer a possibility of measuring the
magnitude of $\lambda_5$.
