Aqueous solutions containing 200 mg/dm3 of p,p'-DDT and methoxychlor were photodegraded for 60 min in UV/TiO2/O2 system and chloride ions concentration and pH were measured. From 60 to 80% of the investigated pesticides were eliminated after treatment. Over 27% of chlorine atoms were splitted off for methoxychlor and 10% for p,p'-DDT. The experimental data suggest, that chlorine atoms were removed from the -CCl3 moiety but the chlorine atoms bound to aromatic ring were left intact at this step of photodegradation.