Apparatus for measuring a reflected light signal on crystalline or powder samples of thermochromic materials, such as 'spin-crossover' materials, has been developed in conjunction with a helium cryostat enabling measurements from cryogenic to ambient temperatures. Depending on the intensity and wavelength of the incident light and optical properties of the sample, reflectivity measurements provide information about a relatively thin surface layer of the investigated compound. The thermal spin transition or photoexcitation from the low-spin state, which is usually coloured, to the high-spin state, which is white or transparent, is detected by a change in the relative intensity as shown on examples of [FexCo1-x(btr)2(NCS)2] · H2O crystalline powders (btr = bis-triazole). To compare the situation in the bulk, simultaneous magnetic measurements were performed. © 1998 IOP Publishing Ltd.