the chemistry of simple phosphole derivatives is comprehensively reviewed to the early part of 1970. using 1,2,5-triphenylphosphole and some of its derivatives as starting materials, several ring expansion and related reactions of the phosphole system have been i studied. thus, pyrolysis and photolysis of 3,4,5- triphenyl-4-phosphabicyclo [3.1.0] hex-2-ene-4-oxide and certain pyrazolino derivatives of 1,2,5-triphenylphosphole oxide lead to the formation of compounds of formula (see document for formula). these compounds are shown to be derivatives of the 4,4’- diphosphabi (cyclohexa-1,5-dienyl) and 4,4'-diphosphabi- (cyclohexa-2,5-dienylidene) systems respectively and possible mechanisms for the formation of these compounds are discussed. furthermore, the first of these compounds may be converted into the second in an unusual reaction in which lithium aluminium hydride acts as an oxidizing agent. the alkaline hydrolysis of 1-iodomethyl-l,2,5- triphenylphospholium iodide is discussed as a possible route to unsymmetrically substituted phosphorins.