The syntheses of cis-and trans-2,2,5-trimethyl-3-phenyl-1,3-oxaphospho~ane are reported and a detailed analysis of the NMR spectra given from which stereochemical assignments were made and conformational structure suggested. Hydroxide cleavage of cis-and trans-3-benzyl-2,2,5-trimethyl-3-phenyl-l,~-oxaphospholanium bromide occurred stereospecifically with retention of configuration a t phosphorus to yield the corresponding diastereomers of 2,2,5-trimethyl-3-phenyl-1,3-oxaphospholane %oxide