Abstract The i.r. spectra (3600-50 cm −1) of the gaseous and solid states, and the Raman spectra (3600-10 cm −1) of the gaseous, liquid and solid states of (CH 3) 2POCH 3, (CH 3) 2POCD 3, (CD 3) 2POCH 3 and (CD 3) 2POCD 3 have been recorded. Thirty-five of the 36 normal modes have been assigned and the asymmetric torsion has been observed as a broad weak feature centered at 106 cm −1 in the far i.r. spectrum of gaseous (CH 3) 2POCH 3. A comparison of the vibrational spectra obtained for the fluid phases with those obtained for the amorphous and annealed solids indicates the existence of a second conformer present in a small amount in the gas phase but becoming more abundant in the liquid phase and the only remaining rotamer in the annealed solid. Asymmetric top i.r. band contour simulation provides evidence that the dominating species in the gas phase is one in which the methoxy methyl group is oriented approximately 60° ( gauche) away from the lone pair of electrons on the phosphorus atom. The rotamer which remains in the spectra of the annealed solid has been assigned to a structure in which the lone pair on the phosphorus atom is oriented trans to the methoxy methyl group. These results are compared to similar data obtained for methoxy phosphoryl compounds and discussed in terms of both canonical molecular orbital and VSEPR thoery.