Phase-change materials, such as the well-known ternary alloy Ge₂Sb₂Te₅, are essential to many types of photonic devices, from re-writeable optical disk memories to more recent developments such as phase-change displays, reconfigurable optical metasurfaces, and integrated phase-change photonic devices and systems. The successful design and development of such applications and devices requires accurate knowledge of the complex refractive index of the phase-change material being used. To this end, it is common practice to rely on published experimental refractive index data. However, published values can vary quite significantly for notionally the same composition, no doubt due to variations in fabrication/deposition processes. Rather than rely on published data, a more reliable approach to index determination is to measure the properties of as-fabricated films, and this is usually carried out using specialized and dedicated ellipsometric equipment. In this paper, we propose a simple and effective alternative to ellipsometry, based on spectroscopic reflectance measurements of Fabry–Perot phase-change nanocavities. We describe this alternative approach in detail, apply it to measurement of the complex index of the archetypal phase-change materials Ge₂Sb₂Te₅ and GeTe, and compare the results to those obtained using conventional ellipsometry, where we find good agreement.