The Cahn‐Fullman method was used to determine three‐dimensional grain size distributions from experimentally measured two‐dimensional chord distributions in high‐purity MgO and magnesiowustite (0.10 and 0.48 wt% Fe2O3). Using the standard deviation of the three‐dimensional distributions, the constant which relates average three‐ and two‐dimensional grain sizes was evaluated by the procedure of Mendelson. The characteristics of the grain size distributions were correlated with steady state, abnormal, and terminal grain growth in MgO and magnesiowustite from 1300° to 1500°C. The distribution parameters were particularly useful in characterizing anomalous (i.e. abnormal and terminal) grain