Rotating heat exchangers (RHEs) transfer contaminants from exhaust to supply air in several ways: with entrained air, through possible leakage around the wheel at the separation wall, and by condensation-evaporation or adsorption-desorption. Leakage from exhaust to supply was measured in several units using the tracer gas dilution technique, and found negligible if not absent in most cases. Leakage can be strongly reduced through a proper installation of the wheel, good maintenance of the gasket, proper installation of a purging sector, and maintaining a positive pressure differential from supply to exhaust duct at wheel level. Pollutant transfer was measured by flash-evaporating a cocktail of 12 chemical compounds in the extract duct, and measuring their concentrations at four points within the air handling unit, upwind and downwind, the heat exchanger in both supply and exhaust flows. The cocktail included diverse alcohols, aldehydes, and cyclic and aromatic hydrocarbons, as well as an ether, an alkane, and a haloalkane. This contribution discusses various transfer phenomena, presents the procedures used to quantify volatile organic compounds (VOCs) transfer and the results of detailed measurements in three units. The transfer rate strongly varies among chemical compounds. In a given chemical compound class, the transfer rate strongly increases with the boiling point. This supports the hypothesis that the main transfer phenomenon is adsorption-desorption.