The five-membered-ring systems germatetrasilacyclopentanes [R2Si]4GeR‘2 (1) (1a, R = i-Pr, R‘ = Me3SiCH2; 1b, R = i-Pr, R‘ = Ph; 1c, R = t-BuCH2, R‘ = Ph) were synthesized and characterized. The structures of 1b,c have been determined by X-ray crystallography. The Si4Ge rings of 1b,c form an envelope and a distorted half-chair conformation, respectively. The photolysis of 1 using two kinds of light sources (low- and high-pressure mercury lamps) was carried out. The photochemical decomposition of 1a afforded two types of four-membered-ring products, cyclotetrasilane (Si4; main) and germatrisilacyclobutane (Si3Ge; minor), with extrusion of germylene (Ge:) and silylene (Si:), respectively. Compounds 1b,c gave, besides the four-membered-ring products as in 1a, two types of three-membered products, cyclotrisilanes (Si3) and disilagermiranes (Si2Ge), with extrusion of silagermene (SiGe) and disilene (SiSi), respectively. This is the first case in which the unusual decomposition of five-membered rings into three-membered rings and dimetallenes has been found.