We investigate the phase behaviors of binary mixtures of polystyrene (PS) hard-sphere and poly(N- isopropylacrylamide-co-acrylic acid) (PNIPAM) soft-sphere colloidal,particles: as a function of temperature; As the temperature increases, apparent attractions arise between the PS particles, to the point of clustering at the highest temperature. This attractive force is found to he due to the change in pH caused by the acrylic acid co-polymerized with the temperature-sensitive PNIPAM particles, which in turn collapses the sterical stabilizing, surface layers on the PS particles. Our to experiments provide a new way to time colloidal interactions with temperature for particles that are otherwise insensitive to temperature variations.