The ultraviolet spectral irradiance has been measured in the Alps in Brian¿on (altitude 1300 m asl) in July 1996 and March 1997, with the spectroradiometer of the University of Lille. The erythemal dose rate has been measured simultaneously with a broadband instrument. The aerosol optical depth, necessary as input for modeling, was measured with a Sun photometer. On cloudy days, irradiance is highly variable, with values exceeding those of clear days when the direct solar beam is not obstructed by clouds. For clear days, measured values are compared with the results of a radiative transfer model, and the influence of altitude and ground surface reflectance is analyzed. For the same solar zenith angle, irradiance in UVA is 8--10% higher in Brian¿on than in Brussels, half of the difference being due to the higher altitude of Brian¿on. Comparison of winter and summer values shows an amplification of ~10--15% in winter owing to the snow reflectance; it can be explained by an effective reflectance of 0.3--0.4. ¿ 1999 American Geophysical Union