This paper describes an automatic weight changer for changing weights on the pan of a mass comparator. The weight changer is easy to use, and is designed to be readily fitted to most common types of top-loading mass comparators and balances of maximum capacity 20 g to 200 g. The versatility of the weight changer is demonstrated by its use in improving the efficiency of routine mass calibration and by its use to compare weights in an evacuated chamber. In each instance the repeatability achieved is 20% or less of the resolution of the balance. The weight changer currently allows up to five weights (or groups of weights) to be automatically compared, but is easily modified to allow comparison of more weights.