The construction of a vacuum microbalance having a sensitivity of 0.3×10−6 g, using a 0.6840‐g sample, is described. The pressure coefficient is found to be less than 0.3×10−6 g for a 1‐atmosphere change. The temperature coefficient is found to be 0.8×10−6 g/°C change. The zero point of the balance is found to be stable to better than 1×10−6 g over a period of 24 hours. The technique of measurements is briefly discussed. Several typical experiments on the formation of oxide films on iron at 25°C and 300°C are presented.