One-colour polarization spectroscopy (PS) on the OH A2S + - X2?(0,0) band has been used to measure the removal of bulk rotational angular momentum alignment of ground-state OH(X 2?) in collisions with He and Ar. Pseudo-first-order PS signal decays at different collider partial pressures were used to determine second-order decay rate constants for the X2?3/2, J = 1.5-6.5, e states. The PS signal decay rate constant, kPS, is sensitive to all processes that remove population and destroy polarization. The contribution to kPS from pure (elastic) alignment depolarization within the initial level, kDEP, can be extracted by subtracting the independently measured or predicted sum of the rate constants for total rotational energy transfer (RET), kRET, and for ?-doublet changing, k?, collisions from kPS. Literature values of kRET and k? are available from experiments with He and Ar, and from quantum scattering calculations for Ar only. We therefore also present the results of new, exact, fully quantum mechanical calculations of kRET and k? on the most recent ab initio OH(X)-He potential energy surface of Lee et al. [J. Chem. Phys. 2000, 113, 5736]. The results for kDEP from this subtraction for He are found to be modest, around 0.4 × 10-10 cm3 s-1, whereas for Ar kDEP is found to range between 0.6 ± 0.2 × 10-10 cm3 s-1 and 1.7 ± 0.3 × 10-10 cm3 s-1, comparable to total population removal rate constants. The differences between k DEP for the two colliders are most likely explained by the presence of a substantially deeper attractive well for Ar than for He. The measurement of kDEP may provide a useful new tool that is more sensitive to the form of the long-range part of the intermolecular potential than rotational state-changing collisions. © the Owner Societies.