31P n.m.r. studies indicate that, in the Mitsunobu reaction, alcohols (ROH) react with triphenyl- phosphine (Ph3P) and dialkyl azodicarboxylates to produce phosphoranes of the type Ph3P(OR)2. Mixtures of alcohols produce, in addition, mixed phosphoranes, Ph3P(OR)(OR'). Phosphoranes of the type Ph3P(OR)N(CO2R?)NHCO2R' are probably intermediates in these reactions but are not observed by 31P n.m.r.