P-​Hydroxytetraorganophosphorane, the long-​postulated intermediate in phosphonium salt and ylide hydrolysis, has been observed and characterised by low temperature NMR, finally definitively establishing its involvement in these reactions.  The results require modification of the previously accepted mechanism for ylide hydrolysis: P-​hydroxytetraorganophosphorane is generated directly by 4-​centre reaction of ylide with water.