Do semiconductor-support interactions influence the bandgap energy? For cadmium sulfide supported onto silica it is shown that the bandgap increases with decreasing coverage. The photocatalytic activity of these heterogeneous photocatalysts increases in the same way as illustrated by the relative rate of an addition reaction. Based on experimental data it is concluded that these changes originate from an electronic semiconductor support interaction (SEMSI) mediated through surface [Si]-O-CdS bonds. It is seen that the effect also changes the flatband potential and charge carrier lifetimes.