The difference in chemical shifts between the two geminal methyl protons of the isopropyl groups of three sulfonium tetrafluoborates free and complexed with Cr(CO)3, has been measured in a variety of solvents. The structure of tricarbonylchromium phenylisopropylmethyl sulfonium tetrafluoborate has been determined by a three-dimensional X-ray analysis. A tentative interpretation of the diastereotopic