AbstractDuring a two-week-long campaign in June/July 2006, samples of volatile organic compounds (VOCs) in ambient air were simultaneously collected at two sites located at different altitudes (500 and 950 masl) in the northern mountainous region of Portugal (Alvão Natural Park), where spring and summer ozone exceedences have been registered over the last years. High ozone concentrations (ca. 150–220 μg m− 3) have been detected in both locations. The calculation of propylene equivalent concentrations showed an important contribution of biogenic VOCs (e.g. 3-carene, isoprene and β-pinene) to the photochemical production of ozone, especially at the low-altitude mountain area. The importance of atmospheric transport on a synoptic scale to the VOC levels in rural mountain areas and the contribution of these precursors to the ozone formation were highlighted.