The response of films to heavy-ion irradiation was investigated systematically by recording dose-response curves for protons and carbon ions in an energy range of 8-430 MeV u(-1). In order to allow for an objective comparison of different experimental results, a practicable method was developed to decouple the value of the net optical density from the film's processing conditions and a precise definition of the film response was deduced. A qualitative explanation of the strong variation of the film response depending on particle type and energy is given, based on the particle's track structure. The data presented here form the basis for using films for dosimetry in mixed particle radiation fields and their application in the heavy-ion therapy project at GSI, Darmstadt.