The influences of surface defects created by hydrogen and oxygen adsorption on the (4 x 1) -> (8 x 2) structural phase transition in an In/Si(111) system have been studied using low-energy electron diffraction. The transition temperature (T-c) decreased with the hydrogen exposure as compared to the clean surface, whereas it increased with the oxygen exposure. The H-induced decrease in the T-c is expected and explainable by the general destructive role of defects as random disorders in the transition. In contrast, the O-induced increase of the T-c is rather unusual. Mobile defects or doping effects may account for the assisting role of the O-induced defects in the condensation of the (8 x 2) low-temperature phase.