We report the preparation, matrix‐isolation, and IR and UV/Vis spectroscopic characterization of phenyl phosphine diselenide, thus providing the first experimental evidence of the monomeric form of Woollins' reagent. Phenyl phosphine diselenide was prepared by thermal dissociation of Woollins' reagent and was identified by matching its spectroscopic data with density functional theory [B3LYP‐D3/6‐311++G(3df,3pd)] computations. The title compound proved to be highly photolabile and irradiation with light at λ = 334 nm results in the formation of hitherto unknown phenyldiselenyl phosphirane. Upon λ = 465 nm irradiation it rearranges back to phenyl phosphine diselenide.