A study of the swelling behaviour of a single batch of high-grade iron ore pellets during hydrogen reduction in a fixed bed showed that the extent of swelling is governed by the nucleation and growth of crystalline iron protuberances from the wstite grains. Over the reduction temperature range 650 to 1000C, it was found that substantial degrees of swelling can occur at temperatures in excess of 850C, and significant variations in the swelling behaviour of individual pellets from different bed levels are observed under these conditions.The results are discussed in terms of a recent swelling theory and shown to be in general agreement.