We report high-resolution infrared absorption spectra of six different CO isotopologues isolated in cryogenic parahydrogen (pH(2)) solids. These data provide a stringent test for theories of nearly free molecular rotors in crystalline solids, such as crystal field theory, rotation translation coupling theory, and the pseudorotating cage model. A gas-phase molecule rotates about its center-of-mass (C.M.); a trapped molecule instead rotates about its "center of interaction" (C.I.) with the trapping cage, which may differ from the C.M. for heteronuclear diatomics like CO. Isotopic manipulation of CO allows the systematic variation of the C.M. relative to the C.I. We report remarkably good straight line correlation plots between the observed matrix effects and C.M. locations. Extrapolation of these lines to the limit of vanishing matrix effects yields an "experimental prediction" of the C.I. in excellent (fortuitous?) agreement with the C.I. calculated using a linear pH(2)-CO-pH(2) toy model.