This report presents a study of the effects of light illumination, from near infrared to blue, on the characteristics of Al/AlOx single-electron transistors (SETs) at low temperatures (0.3-4.2 K). Several effects on the SET conductance are observed when the devices are subjected to light illumination, including changes in the Coulomb blockade oscillation period and amplitude. To determine the origin of the observed effects, SETs with different device geometries were fabricated on semiconducting and insulating substrates. The results show that illumination of semiconducting (Si) substrates leads to the excitation of mobile carriers at the insulator-semiconductor interface that strongly influence the SETs, while the use of wide bandgap insulating substrates (quartz) enables SET operation that is immune to visible light illumination from incident powers of 3 mu W/mm(2) (flux of about 10 photons/nm(2) s for whole visible light spectrum).