A new method of reflectivity measurement for the study of reflective materials in solar energy technology is reported in this paper. The method employs a fast rotating reference mirror and certain geometry configuration to alternatively deliver the light via a sample mirror or by passing it to a photo detector. It has been proved to reach a precision of 10(-4). With a good repeatability of the method, outdoor exposure tests of sample mirrors have been carried out in a relatively short period (1 month) and significant specular reflectance losses due to an aging process have been observed. (C) 2003 Elsevier B.V. All rights reserved.