The photoinduced refractive index change of polyester with disperse red side groups was studied at several different wavelengths. This material exhibits a large photoinduced index change (0.3 at 633 nm) and in addition when poled has a sizable second‐order nonlinear optical effect. This index change was monitored over 1000 h and long‐term stability was demonstrated. Based on these results, a method using only photomasks to define channel waveguides and other patterns in such polymers was suggested and experimentally demonstrated. This photoinduced index change may have wide applications in integrated optical systems when these or other similar polymers are involved.