The Metropolitan Region of Santiago de Chile frequently faces high concentrations of ozone and peroxyacetyl nitrate (PAN). During spring time of the year 2002 a field experiment was carried out in this area focusing on three representative monitoring sites located along an axis through this area that allowed to observe upwind, downtown and downwind conditions. Three consecutive days reflecting different photochemical and meteorological conditions were selected. These days included two workdays and one holiday and thus the effect of different primary emissions could be investigated. A variety of trace gas measurements (O3, NOx, CO, volatile organic compounds (VOC)) were obtained at these sites. Based on absolute values the results show that the alkanes represent the largest VOC fraction at all sites, the second one being the aromatics and the third one being the alkenes. The smallest fractions are represented by the alkines or isoprene. Studies of the propene equivalent show a temporal sequence in terms of photochemical impacts of the different VOC fractions: during morning hours the alkenes are the most important ones. At noon aromatics are dominant, and in the afternoon isoprene becomes very important. On the whole, alkanes - though being present at high concentration levels mainly due emissions of liquefied petroleum gas (e.g. propane) - do not contribute more than 20% to the total air mass reactivity. Observations from this campaign were also compared with numerical simulations. These simulations also indicate significant amounts of secondary organic aerosols. Anthropogenic hydrocarbons are expected to be then main precursors.