We study the apparatus response function g (t) which must be known in order to deconvolute the fluorescence decay experiments obtained by the single photon counting method. Due to the flash and the PM characteristics, g (t) depends on the excitation and emission wavelengths. The PM contribution is analyzed qualitatively and found to be in agreement with the properties of the photoelectric effect. Therefore, the accurate determination of the ``true fluorescence decay'' cannot be obtained by the process commonly used, which consists in replacing g (t) by the apparatus response obtained with the PM receiving the light of the exciting pulse. The function g (t) may be obtained in using a reference fluorescent solution with a known fluorescence decay-time.