The oxidation capacity of the highly polluted urban area of Santiago, Chile has been evaluated during a summer measurement campaign carried out from 8-20 March 2005. The hydroxyl (OH) radical budget was evaluated employing a simple quasi-photostationary-state model (PSS) constrained with simultaneous measurements of HONO, HCHO, O-3, NO, NO2, j((OD)-D-1), j(NO2), 13 alkenes and meteorological parameters. In addition, a zero dimensional photochemical box model based on the Master Chemical Mechanism (MCMv3.1) has been used to estimate production rates and total free radical budgets, including OH, HO2 and RO2. Besides the above parameters, the MCM model has been constrained by the measured CO and volatile organic compounds (VOCs) including alkanes and aromatics. Both models simulate the same OH concentration during daytime indicating that the primary OH sources and sinks included in the simple PSS model predominate. Mixing ratios of the main OH radical precursors were found to be in the range 0.8-7 ppbv (HONO), 0.9-11 ppbv (HCHO) and 0-125 ppbv (O-3). The alkenes average mixing ratio was similar to 58 ppbC accounting for similar to 12% of the total identified non-methane hydrocarbons (NMHCs). During the daytime (08:00 h-19:00 h), HONO photolysis was shown to be the most important primary OH radical source comprising alone similar to 55% of the total initial production rate, followed by alkene ozonolysis (similar to 24%) and photolysis of HCHO (similar to 16%) and O-3 (similar to 5%). The calculated average and maximum daytime OH production rates from HONO photolysis was 1.7 ppbv h-(1) and 3.1 ppbv h(-1), respectively. Based on the experimental results a strong photochemical daytime source of HONO is proposed. A detailed analysis of the sources of OH radical precursors has also been carried out.