The influence of the copolymer molar mass on the thermoreversible gelation of the triblock copolymer polystyrene-b-poly(ethylene/butylene)-b-polystyrene in a paraffinic oil was studied. The solvent used is selective for the middle poly(ethylene/butylene) block of the copolymer. The thermal behavior as a function of copolymer concentration was investigated. The sol-gel transition temperature increased with the copolymer concentration and molar mass. The mechanical properties of the gels were examined through oscillatory shear and compressive stress relaxation measurements. The concentration dependences of the elastic storage modulus were established and were observed to be similar for the three copolymers. The relaxation rates observed were extremely high, indicating a considerable mobility in the gel over the measurement time. The relaxation rate decreased as the copolymer molar mass increased. The swelling behavior has also been examined. All gels swelled significantly The copolymer concentration dependences of the swelling ratio were linear but their ordinates were not close to unity. Whereas the two copolymer with higher molar mass show a similar swelling behavior, the copolymer with the lowest molar mass shows a swelling capability markedly smaller.