A series of photocrosslinkable, side-chain, second-order nonlinear optical (NLO) poly(ester imide)s (PEIs) based on a chromophore-containing dianhydride, 2,2'-[4- [(4-nitrophenyl)-azolphenylliminobis(ethyl benzene-1,2-dicarboxylic acid anhydride-4-carboxylate), benzophenone-3,3',4,4'-tetracarboxylic dianhydride, and 4,4'-diamino3,3'-dimethyl diphenylmethane were prepared. The resulting PEIs exhibited many useful physical characteristics, such as good organosolubility, excellent film-forming properties, high glass-transition temperatures (186-229 degreesC, and high thermal decomposition temperatures. The electrooptic coefficient value of PEI3 at 650 nm was 11.5 pm/V, and high long-term stability of the NLO chromophore alignment in the poled PEI3 film at 120 degreesC was observed. The temporal stability of the dipole orientation at 150 degreesC was further enhanced by ultraviolet irradiation because of photocrosslinking.