The catalytic reactions between acetone vapour and deuterium on evaporated metallic catalysts in a static system have been followed by means of a mass spectrometer. Reduction of the carbonyl group and exchange of the methyl groups have been observed on films of rhodium, palladium, platinum, nickel, iron, tungsten, gold and silver. The reaction of acetone with hydrogen on evaporated metal films (Stoddart & Kemball 1956) has shown that the main reduction product was isopropanol, and the use of deuterium has now established that the mechanism of this reduction involves the simple addition of two 'hydrogen' atoms to the carbonyl bond. Rates of deuteration to isopropanol were generally less than rates of hydrogenation, and this effect has been accounted for in terms of zero-point energy differences between the two isotopes. Energies of activation and frequency factors have been obtained for the exchange reaction on the various metals and mechanisms of exchange have been deduced. Evidence has been obtained of the inhibition of the exchange reaction by the isopropanol produced in the reduction, and a modified exchange equation has been derived for the general case when a reduction reaction accompanies exchange. A correlation has been obtained between the activity of the metals for the exchange of acetone and the percentage d-character of the metallic bonds.