Spectroscopic and theoretical studies were carried out for O,O,O-trimethyl selenophosphate molecule. DFT structural and vibrational calculations were performed at 6-311++G∗∗ level. Ar/matrix-FTIR spectra were recorded. A coexistence of different conformers with C3 and C1 symmetries was detected at different temperatures. Spectral evidence of a lower energy Cs conformer was found. These conclusions are consistent with the results from DFT calculations. A tentative assignment of the features observed in the Ar/matrix-FTIR spectra is proposed.