The pulse characteristics of a photolytic, gain-switched iodine laser are a sensitive measure for relaxation processes that occur in the active medium. On a time scale shorter than a few microseconds this concerns the relaxation of the speed and the angular parts of the anisotropic velocity distribution which is produced by the photodissociation of the parent molecule CF3I. A comprehensive rate equation model of the laser dynamics is fit to experimental data, to obtain time constants for these collision-controlled relaxation processes for various buffer gases and pressures.