Abstract The infrared (3500–80 cm−1) and Raman spectra (3500–40 cm−1) of gas/or liquid and solid (CH3)2PX with X = H (DMH), CN (DMCN) and Cl (DMCl) as well as (CD3)2PH have been recorded and complete vibrational assignments are given for all three molecules. To support the spectroscopic study, ab initio calculations by the Moller–Plesset perturbation method to second order MP2(full) and density functional theory calculations by the B3LYP method have been carried out. The infrared intensities, Raman activities, vibrational frequencies and band contours have been predicted from MP2(full)/6-31G(d) calculations and these theoretical quantities are compared to experimental ones when available. By utilizing the previously reported microwave rotational constants for DMH and DMCN along with the MP2(full)/6-311+G(d,p) predicted values, adjusted r0 structural parameters for DMH and DMCN have been determined. The heavy atom parameters for DMH are: r0(P C3,4) = 1.8477(30) A, ∠ CPC = 99.88(50)° and for DMCN: r0(N C) = 1.159(3), r0(C P) = 1.790(3), r0(P C4,5) = 1.841(3) A, ∠ NCP = 175.7(5), ∠ CPC4,5 = 97.9(5) and ∠ CPC = 100.7(5)°. Barriers to internal rotation are reported. The experimental values are compared to the corresponding values of some similar molecules whenever possible.