Abstract Macor machinable glass-ceramic was irradiated to fluences up to 1 × 10 23 14 MeV n/ m 2 at room temperature. Post-irradiation measurements were carried ut to determine changes in high-frequency electrical conductivity, hardness, and density. It was found that neutron damage caused slight increases in conductivity and hardness. The major change noted was in density, where a fluence of 4 × 10 22 n/ m 2 caused swelling of 1.55 vol% while a dose of 1 × 10 23 n/ m 2 resulted in a lower swelling value (0.82 vol%). This unusual behavior is explained by a model involving expansion of the mica phase of Macor and contraction of the glassy phase. Implications of the present results for engineering performance of Macor at these and higher fluences are discussed.