Photocatalyzed decompn. of 1, 1,3,3-tert-butylphenot ethoxylates (C8H17(C6H4)O(C2H4O)(n)H in wastewaters was pre-studied via adsorption from aq. 120 or 200 Triton X-100 (n(av) = 9.5) and 40 or 80 mg/l. X-45 (n(av) = 4.5) solns., each on 1.5 g/l. pure (or 0.5% Pt-activated) TiO2 (similar to 50), 0.1-0.2-mm SiO2 (428) or 10%TiO2-on-SiO2 (382 m(2)/g) at room temp. in partial darkness. On TiO2, equil.(similar to 15% of the initial X in soln.) was established in 50 min; Langmuir's isotherm was obeyed and the equation consts. detd. for each X adsorbed on TiO2 were close to each other. The rate, equil. const., and coverage x = 0.11 mol. X/nm(2) were not related to oxyethylene chain length nav, At concns. exceeding the CMC (= 150 and 70 mg/l. for X-100 and X-45, resp.), no adsorption occurred. On SiO2, x = 0.107 and 0.04 mol. X-100 and X-45/nm(2), resp., showing chain length n(av) likely to act The small x-values suggested adsorption to affect photocatalytic degradation of nonionics only slightly.