The temperature dependence of the refractive indexnof cubic crystals undergoing phase transitions (PT) is measured with an accuracy of the order &dgr;n=10−6at temperatures between 3.5 and 1000 K with a computer‐controlled Jamin–Lebedeff interferometer. Microscopic monitoring forinsitusample orientation and single domain work is provided. Results are presented for KMnF3(antiferrodistortive PT) and NiO (antiferromagnetic PT) and discussed within the framework of a generalized phenomenological indicatrix perturbation theory. It predicts the proportionality between the order parameter autocorrelation function ⟨&dgr;&eegr;2⟩ and the refractive index anomaly preceding the PT. Comparison with recent theoretical predictions is performed.