Exposure of sulfonated poly(ether etherketone), SPEEK, in aqueous solutions to 350 nm photons induced reduction of CHCl3 to CH2Cl2 and chloride ions in the presence of HCO2H/HCO2- buffers or poly(vinyl alcohol), PVA. The kinetics of the SPEEK-sensitized photoreaction was characterized by quantum H yields of halide ion formation, phi(Cl-), evaluated from in situ determinations of [Cl-]. Particularly efficient reductions took place when formate buffers served as H atom donors in the absence of air and with excess CHCl3. The dependence of phi(Cl-) on the inverse square root of the light intensity together with postirradiation formation of Cl- in the dark indicated that the CHCl3 photoreduction occurred via a chain process. EPR determinations identified the alpha-hydroxy radical of SPEEK and (CHCl2)-C-center dot as chain carriers. Most of the kinetic findings were rationalized in terms of a free radical mechanism where dimerizations of the radicals acted as termination steps. Photoreduction of CHCl3 was also detected in the presence of air albeit with lower quantum efficiencies. Observations made during postirradiation experiments indicated that a chain process was also operative under such conditions.