In this study the optimum tilt angle for south-facing solar surfaces in Tabass, Iran, for the fixed monthly, seasonal, semi-yearly and yearly adjustments were calculated. Due to lack of measured diffuse solar radiation data, to predict the horizontal diffuse radiation nine diffuse models from three different categories were established. Based on some statistical indicators the three degree model, in which both clearness index and relative sunshine duration are variables, was recognized the best. The monthly optimum tilt varies from 0 degrees in June and July up to 64 degrees in December and the yearly optimum tilt is around 32 degrees, which is very close to latitude of Tabass (33.36 degrees). For different adjustments, particularly for a vertically mounted surface, the received monthly mean daily solar radiation components and the annual solar energy gains were calculated and compared. Total yearly extra solar gain for the monthly, seasonal, semi-yearly and yearly optimally adjusted surfaces compared to that of horizontal surface are 23.15%, 21.55%, 21.23% and 13.76%, respectively. The semi-yearly tilt adjustment of 10 for warm period (April-September) and 55 for cold period (October-March) is highly recommended, since it provides almost the same level of annual solar energy gain as those of monthly and seasonal adjustments. (C) 2013 Elsevier Ltd. All rights reserved.