Abstract Daily radiation on slopes in usually computed by the Liu and Jordan method. Measurements of the global and diffuse radiation fluxes at various tilt and azimuth angles have been made in clear days with a pyranometer equatorially mounted upon a modified theodolite system. The resulting data have been used to evaluate the validity of the Liu and Jordan model with the assumption of an isotropic distribution of diffuse radiation in the sky. It is concluded that the model yields significantly better results when the reception surface is oriented in the azimuth of the Sun, but not for other azimuthal orientations of the slope. Although an evident anisotropy has been found in the diffuse radiation distribution, the results obtained by this modeling approach are considered sufficient for flat-plate collector applications.