Onboard calibration of radiometric measurements in remote sensing is often achieved by observation of a sun-illuminated reflectance secondary standard using previously calibrated reflecting diffusers. In the calibration procedure, information such as the bidirectional reflectance distribution function (BRDF) of the diffuser outside the measured calibration set, or a model of the degradation of the diffuser performance in space conditions, is required. We have modeled the measured BRDF calibration data of the Spectralon® diffusers used in the Medium Resolution Imaging Spectrometer (MERIS) currently operated as part of the Earth-observing satellite ENVISAT. The diffuser is modeled by a layered structure where the light transport in the layers is described by radiative transfer theory. The BRDF is characterized by a limited number of parameters describing the optical properties of the material, e.g., albedo, optical thickness, and anisotropy of the scattering. We also describe the result of modeling the degradation of a Spectralon diffuser after deposition, under UV radiation, of a silicone layer.