Satellite observations over the ocean in the backscatter direction are dominated by Rayleigh scattering. We use this predictable radiance for in-flight calibration of the visible SPOT channels. Two methods are evaluated. The first method directly relates the measured numerical signal in a short wavelength channel to the predicted reflectance. In the second method, we use a second channel centred at a longer wavelength, to correct the short wavelength channel for the effect of the atmospheric aerosol contribution. These two methods are examined for channelB0, centred at 045μmplanned for launch on SPOT-4 VEGETATION, and for channelB1centred at 0-55mu;mm, currently on-board SPOT-1 HRV. In both cases, the channelB3, centred at 0-85mu;mmis used for aerosol correction. Error analysis shows that accuracies of 3 and 5 per cent respectively can be achieved for B0and B1. The last section of the paper is devoted to a validation of the error analysis using SPOT-1 HRV data.