The photoinduced reaction of disilane on Si(lll) using ultraviolet (UV) irradiation has been studied using temperature-programmed desorption (TPD). Hydrogen and silane desorption yields were used to determine the reactivity of disilane on Si(lll) with and without UV irradiation. Photoinduced reaction of disilane was accomplished either through simultaneous exposure of disilane and UV irradiation or by UV irradiation of a physisorbed disilane layer after exposure to disilane. These two methods of photoexcitation were compared to the thermal reaction of disilane with clean Si(lll). UV irradiation during or after dosing of the crystal surface at 110 K greatly enhanced the reactivity of disilane on Si(111) compared to similar disilane exposures without UV irradiation. UV irradiation during disilane exposure of Si(lll) at low temperatures results in greater Si deposition than does UV irradiation following disilane exposure. The mechanism of the photoinduced disilane reaction has been studied using clean, partially deuterated, and fully deuterated Si(lll) surfaces. Thermally, the reactivity is controlled by the surface dangling bonds, whereas the photoinduced chemistry appears to primarily involve insertion by the photogenerated diradical, silylene. Evidence was found for the existence of a molecular physisorbed precursor state that interacts directly with the dangling bonds of Si(111).