Radical copolymerization of a 4:1 mixture of methacrylic acid and N-[4-(phenylazo)-phenyl]methacrylamide afforded a copolymer of methacrylic acid bearing 10.4 mol% pendent azobenzene units. The copolymer was sensitive to irradiation by virtue of the photochemical trans-to-cis isomerization of the pendent chromophore. The pH-dependent conformational transition characteristic of poly(methacrylic acid) was shifted to a higher pH by the hydrophobic nature of the azobenzene units, but viscosity measurements revealed no further shift in the transition upon photoisomerization. The potentiometric titration curves of the two photoisomeric states of the chain were displaced in a manner consistent with the enhanced polarity of the cis form of the chromophore. The adsorption of the chain on phosphatidylcholine bilayer membranes was strikingly sensitive to light; at pH 7, the cis form adsorbed weakly if at all, whereas the trans form bound strongly to the membrane surface with concomitant reorganization of the lipid bilayer structure.