"Laser snow" or laser-induced clustering describes a process whereby laser irradiation of a moderate- to high-pressure molecular gas results in the visible precipitation of particulates. We report a molecular beam analog of laser snow where photochemical polymerization occurs within molecular clusters rather than via a collisional mechanism. Clusters of CS2 are formed in a supersonic expansion with argon and photoionized by two-photon absorption of 239.53 nm photons. Various ionic photoproducts such as S-m(+), S-m(+)(CS2)(n), (CS)(m)(+), and (CS2)(m)(+) clusters are detected by mass spectrometry. A cycle of ion-molecule reactions occurring entirely within an isolated molecular cluster is proposed to explain the observed ions.