Structure, crystal growth kinetics, and rigidity of thermosensitive colloidal crystals formed from mixtures of colloidal silica spheres (diameter, 103 nm) and gel spheres of poly(N-isopropylacrylamide) are studied mainly from reflection spectroscopy. The gel spheres change their hydrodynamic size transitionally at 35 degreesC from. 72 nm (at 20 degreesC) to 28 nm (at 45 degreesC). The mixtures are deionized exhaustively with ion-exchange resins in aqueous suspensions. In the absence of gel spheres, intersphere spacings of the crystals, l(obs), are quite insensitive to suspension temperatures ranging from 15 to 55 degreesC. In the presence of the gel spheres, on the other hand, the lobs value decreases transitionally at 35 degreesC when suspension temperature increases. The zeta-potential and the effective diameter of colloidal silica spheres determined by electrophoretic light-scattering measurements increase as the gel concentration increases. Addition of the gel spheres decreases the crystal growth rate and increases the rigidity of the mixtures. These results support strongly that the thermosensitive gel spheres adsorb weakly onto the surface of colloidal silica spheres.