Low‐loss dielectric optical waveguides have been made by a new process in which a photochemical reaction is used to lock or fix a dopant in a polymer film with a lower refractive index, and unreacted dopant is then removed by heating. Narrow guides ([inverted lazy s]4 &mgr;m wide, &Dgr;n[inverted lazy s] 1%) were formed by laser beam writing at &lgr;=364 nm or by contact printing at &lgr;=313 nm, and had losses of 0.20±0.05 dB/cm at &lgr;=633 nm.