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A device has been developed for the accurate weighing of 20 specimens, using a single electronic microbalance. The balance output is interfaced to a data logger and a computer is used to process the results. The operation of the equipment and the computing is described. The results of several long duration experiments on mild steels in CO2-based gas at temperatures up to 770 K and pressures up to 2.8 MNm−2 are discussed particularly in terms of accuracy and reproducibility and the effects of temperature and hydrogen on the post breakaway rate. For an inert Ni specimen at 770 K and 2.8 MNm−2 the standard deviation of the weight determined over 200 and 800 h. at 1.5 h. intervals was found to be 43 and ∿ 60 μg respectively. For mild steel samples an oxidation rate of 5 μg cm−2h−1 could be determined to ± 5% at the 95% confidence level in an experiment of 200 h. duration. The oxidation rate of a mild steel in these tests was unaffected by graphite paint or by contact with Ni and it compared closely with results obtained by intermittent weighing methods.