AgBr microcrystals that have been treated with reducing agents to form small silver clusters on their surface have been examined with low-temperature optical and optically detected magnetic resonance methods. These investigations have indicated that these treatments produce two closely related Ag clusters that are spectroscopically active. One of the centers manifests itself by low-temperature emission bands at 550 and 640 nm and an absorption band at 442 nm. The other center, which absorbs at 430 nm, acts as a surface hole trap that completely changes the nature of the AgBr donor acceptor recombination. These centers are only observable on crystals with (111) faces. Spectroscopic and theoretical considerations suggest that these spectroscopically prominent centers are silver dimers in two different surface configurations.