Photoablation of Polystyrene (PS) and Polymethylmethacrylate (PMMA) by 193 nm radiation of an excimer laser was studied in two types of experiments. 1) Time resolved absorption measurements show that the ablation of PS starts within the first few nanoseconds during the laser pulsa. 2) By laser-mass-spectrometry of the ablated neutral species their chemical nature and velocity distributions were measured. At low fluences, monomer molecules are the main product, at high fluences many smaller fragments are observed. The velocity distributions of the molecules are similar to that of adiabatic gas expansions. For comparison experiments with condensed CH3I were performed. Extremely sensitive detection of CH3I, CH3 and I by resonant multiphoton ionization combined with time-of-flight (TOF) mass-spectrometry allows to distinguish CH3 radicals from "direct" dissociative UV-photodesorption, "thermalized" dissociation products and thermal desorption of parent molecules at low fluences. The more efficient, "explosive" photoablation is observed at fluences above 1,5 mJ/cm2.