This paper presents the performance of 10 arithmetic models used to estimate diffuse solar irradiance on inclined surfaces in a comparative study with actual data readings made available on an hourly and a daily basis. The data readings have been taken from a south facing surface inclined at 42 degrees in an area at some distance from the provincial capital in the Spanish province of Valladolid. In order to confirm the results, three statistical parameters have been used in the study; root mean square error (RMSE), mean bias error (MBE) and Stone's t-statistic. The results obtained favour the Muneer model, followed by the Reindl model, for hourly as well as for daily values. The Temps-Coulson model gives rise to great discrepancies with respect to the values measured. The results for the Perez model are not good due to the use of parameters that are not specifically calculated for the area in this study, which underlines the need to take an area's features into account so that predictions for diffuse irradiance measured on inclined surfaces may be as accurate as possible. (c) 2004 Elsevier Ltd. All rights reserved.