The interconversions of isomeric esters of 3-methylcatechol and 1-hydroxyphosphetan 1-oxides (phosphetinic acids) have been studied by variable temperature ¹H n.m.r. spectroscopy (ΔG*ca. 17–18 kcal mol^–1) and the intermediate hydroxyspirophosphoranes trapped as methoxyspirophosphoranes on treatment with diazomethane. The more stable ester from 3-methylcatechol and diphenylphosphinic acid has been obtained essentially pure; it isomerises to the less stable isomer with ΔG* 24.4 ± 0.2 kcal mol^–1. No methoxyphosphorane was detected on treatment with diazomethane. The corresponding phosphinothioate esters have higher free energies of activation for interconversion and the intermediate hydrothiospirophosphoranes are not trapped with diazomethane. P-Hydroxy-2,2′-spirobi-(1,3,2-benzodioxaphosphole) has been obtained in solution in THF by treatment of the corresponding P-chloro-compound with one mole equivalent of water. With chlorotrimethylsilane and triethylamine it gave the P-trimethylsilyloxyphosphorane; with o-hydroxyphenyl o-phenylene phosphite (36) in DMF it gave the tris-(o-phenylene) phosphate anion (37) and o-phenylene phosphonate.