The optical absorption spectra of dilute silver iodide hydrosols containing 1-phenyl-5-mercaptotetrazole (PMT) in concentrations comparable to those of the iodide ion showed that the coexistence of PMT at the stage of sol preparation favored the appearance of the absorption shoulder at ca.445 nm characteristic of α-form (high-temperature cubic) silver iodide, while both this band and the band at ca. 420nm characteristic of low-temperature silver iodide (mixture of hexagonal β-form and cubic γ-form) disappeared on prolonged aging of the sols. These observations suggested a relatively strong interation between PMT and silver iodide. X-ray diffractometry of coprecipitated and aged mixtures of silver iodide and silver phenyl-mercaptotetrazolate revealed the existence in them of a new adduct or compound Ag₂I (PMT).