Hourly global irradiations on tilted planes are required in various engineering calculations for solar systems. In a lot of sites, at best, only global irradiations on horizontal planes are available. To calculate the global irradiation on inclined surfaces from horizontal global irradiation, the fist step consists in determining the horizontal diffuse component. Thus, we study the variation of the diffuse component with global irradiation on an hourly basis. Several correlations between hourly values of diffuse and global irradiations are presented, validated and compared using solar data collected on the French Mediterranean site of Ajaccio. (C) 2004 Elsevier Ltd. All rights reserved.