The Ultraviolet Index (UVI) obtained by a multichannel radiometer GUV‐2511 (Biospherical Instruments Inc.) is analyzed in this paper. This instrument has been recently installed in the radiometric station of Extremadura University in Badajoz (Spain). The UV Index has been calculated by a linear combination of three and four GUV channel irradiance values. In order to test these results, simultaneous values of UVI were calculated with the data provided by a well calibrated broadband UVS‐E‐T instrument colocated side by side the GUV multichannel. This radiometer has a spectral response that is adapted to the erythemal (sunburn) action spectrum of the human skin. One‐minute simultaneous values of both instruments have been used to calculate ultraviolet Index (UVI). Although the measurement period is limited, it covers all sky conditions, from cloud‐free to overcast days. UVI calculated with the whole data set by both methods are well correlated, but not as well as using only clear day data. It was observed that GUV underestimates UVI values (mean bias error, MBE=18.9%), being the four channel method the most successful.