Novel lactamimide-containing polymers were first synthesized by the bisimide-lacramimide ring contraction of 1,8:4,5-tetracarboxylic bisimide-containing polymers. The conversion of bisimide moieties was in the range 68-100% depending on polymer chain flexibility. The yields of lactamimide units were between 75 and 100% with respect to bisimide group conversion owing to the insolubility of polymers during lactam ring closure. No appreciable hydrolytic degradation of polymers was observed as followed from GPC data. Both u.v. long-wave absorption and emission maxims in polylactamimides were 20-50 nm red shifted compared with those of polyimides due to intramolecular charge transfer. It was found that the thermostability of lactamimide units was close to that of bisimide as proved by TGA.