Phosphorin 1-sulfides generated by the thermolysis of 1-phospha-2-thiabicyclo[4.4.0]deca-3,7,9-triene 1-sulfides reacted with triphenylphosphine, dienophiles, or nucleophiles to give the corresponding phosphorins, 1-phosphabicyclo[2.2.0]octa-2,5-diene 1-sulfide, or 1,2-dihydrophosphorin 1-sulfide derivatives, respectively.