We report the effect of microphase-separated structure on the mechanical and thermal properties of several poly(1,3-cyclohexadiene-block-butadiene-block-1,3-cyclohexadiene) triblock copolymers (PCHD-block-PBd-block-PCHD) and of their hydrogenated derivatives: poly(cyclohexene-block-ethylene/butylene-block-cyclohexene) triblock copolymers (PCHE-block-PEB-block-PCHE). Both mechanical strength and heat-resistant temperature (ex. Vicat Softening Temperature: VSPT) tended to increase with an increase in the 1,3-cyclohexadiene (CHD)/butadiene ratio. On the other hand, heat resistance of the hydrogenated block copolymer was found to be higher than that of the unhydrogenated block copolymer. However, the mechanical strength was lower than those of the unhydrogenated block copolymer with the same ratio of CHD to butadiene. To clarify the relationship between the higher order structures of those block copolymers and their properties, we observed the microphase-separated structure by transmission electron microscope (TEM). Hydrogenated block copolymers were found to have more finely dispersed microphase-separated structures than those of the unhydrogenated block copolymers with the same CHD/Bd ratios through the use of TEM and the small-angle X-ray scattering (SAXS) technique. Those results indicated that the segregation strength between the PCHE block sequence and the FEB block sequence increased, depending on hydrogenation of the unhydrogenated precursor. (C) 2000 John Wiley & Sons, Inc.