Saturated carboxy- and hydroxy-terminated hydrocarbon prepolymers and selected aziridine, epoxide, and isocyanate curing agents were evaluated with the purpose of preparing an elastomeric binder for solid propellants. Viscosities at a given molecular weight were higher for polyisoprenes than for polybutadienes and higher for carboxy terminated than for hydroxy terminated. Isocyanates showed much less side reactions in the presence of ammonium perchlorate than did the aziridines or the epoxides. The polymers had glass transition temperatures between -40° and -82°C when measured by density change. Differential thermal analysis indicated two secondorder transitions at about -64° and -38° for the saturated prepolymers and single transitions at about -75° and -90°C, respectively, for unsaturated prepolymers and polymers made from them. The polymers showed no tendency to crystallize. All of the polymers and propellants showed poor low-temperature behavior. The poor low-temperature properties have impeded acceptance of these materials by the industry. Partially saturated prepolymers show considerably improved low-temperature properties, indicating that saturated hydrocarbon chains are less flexible than the corresponding unsaturated one.