The assignments of several infrared bands of aliphatic phosphorus compounds have been reviewed and extended on the basis of new or improved data. These bands include POP, P → S, PSH, PSR, PH, and PCl. Several new assignments are proposed, mainly in the 12–24 μ region. Two examples are given of the application of these assignments to the determination of the structure of the thiopyrophosphates. It is shown that all procedures used for preparing monothiopyrophosphates yield exclusively the thiono isomer.