Poly(styrene-co-N-isopropylacrylamide) core–shell microspheres have been synthesized by a two-step surfactant-free emulsion polymerization process. These microspheres are thermo-responsive with a clear volume phase transition around 30 °C and form crystalline colloidal arrays over a wide range of concentrations. The resulting crystalline arrays also display a response to temperature, visible through a change in the diffraction spectrum. However, the response appears to be highly dependent on the microsphere concentration. This behavior originates from a change in the short-range interactions between particles rather than from the volume change of the particles caused by the volume phase transition. Attractive van der Waals interactions increase while repulsions decrease; these changes only affect the equilibrium of concentrated samples, where the interparticle distance is sufficiently short to have an effect.