Photolysis of the silyl-substituted 1H-phosphirene 3 a proceeds selectively with cleavage of a silicon ± silicon bond and ring expansion to furnish the 1,2-dihydro-1,2-phosphasilete 4 a. The corresponding heterocyclic germanium product, 4 b, is prepared analogously from 3 b. The preparative significance of 4 a is reflected not only in its numerous addition reactions to multiple-bond systems, such as alkynes 8 and 12 and ketene 10, but also in its substitution reactions with the chloro compounds 15, 18, and 21. The latter reactions proceed through the formation of chlorotrimethylsilane and the novel 1,2-dihydro-1,2-phosphasiletes (16, 19, and 22) which, depending on the substitution pattern at the phosphorus atom, undergo various isomerization reactions (16 317, 19 320). The hydrolysis of 4 a to 23 by mere traces of water in carefully purified tetrahydrofuran demonstrates the extreme sensitivity of this compound towards moisture.