Two kinds of quantized PbS microcrystals modified with 4-aminothiophenol (4-ATP/Q-PbS) having different surface coverage were prepared by using two different concentration ratios of [Pb2+]/[S2-] in the synthesis of the 4-ATP/Q-PbS particles. The rates of photoinduced reduction of methylviologen on the two kinds of 4-ATP/Q-PbS particles were influenced by the kind of hole scavengers (sodium tartrate or triethanolamine) and the pH of solutions, and the higher reduction rates were obtained under conditions where the higher amount of adsorption of the hole scavenger occurred. High reduction rates of methylviologen were obtained at the photocatalyst having a high coverage of the surface modifier, because high electrostatic attractive interactions between the hole scavenger and the surface are operative when tartrate was used as a hole scavenger and a high amount of adsorption sites due to a high coverage of the surface modifier are available when triethanolamine was used. The validity of such interpretation was supported by kinetic analysis based on rate equations. Furthermore, it was found that inert electrolyte anions such as ClO4- had a retarding action for the photoreduction of methylviologen when tartrate was used as the hole scavenger, as a result that the adsorbed tartrate anions were replaced by the inert electrolyte anions depending on their concentrations. The adsorbability of inert electrolyte anions (Cl-, Br-, and ClO4-) as well as tartrate anions was found to be influenced by the degree of surface modification of the Q-PbS particles with 4-aminothiophenol. As for the case of using electronically neutral triethanolamine as the hole scavenger, the inert electrolyte anions did not influence the photoreduction rate of methylviologen.