The response function of 2-cm-thick CsI(TI) scintillators with photodiode readouts were studied by directly exposing the detectors to beams of heavy ions (2 less-than-or-equal-to Z less-than-or-equal-to 36) with energy up to 25 MeV/u. The dependence of the light output on the energy (E) as well as on the atomic number and the mass of the ion is analyzed and discussed, and a parameterization of the light output as a function of Z and E is proposed.