The picosecond evaporation dynamics of ammonia molecules from Na(NH3)(n) clusters produced via the dissociation of NaI within an ammonia cluster is studied experimentally. A time dependence of the mass selected ion signal is observed only for one cluster size and not for larger clusters nor for smaller ones. It will be shown that evaporation in the ionic state is responsible for the absence of an observation of the evaporation in the neutral state. The variation of the ionization cross section with cluster size is the key point for the interpretation of the results.