A vacuum microbalance technic for the study of chemical reactions occurring on metallic surfaces is described. Preliminary curves are presented for the oxidation of pure electrolytic iron from 25° C. to 400° C. and at various pressures of oxygen. Attempts are made to correlate the data with the various mechanisms proposed for oxidation reactions. A suggestion is made that the overall rate may be thought of as a sum of three individual rates. These are: (1) a rapid initial rate; (2) an intermediate reaction which involves the logarithm of the difference between an extrapolated parabolic law and the experimental value as a function of time; and (3) a parabolic law which is found to hold above a certain temperature and film thickness.