The relationships of monoterpene emission with temperature, light, photosynthesis and stomatal conductance (g(s)) were studied in Quercus ilex L. trees throughout the four annual seasons under field conditions. The highest monoterpene emission was measured in spring and summer (midday average of 11 μg [g DW]-1 h-1), whereas the lowest rates were found in autumn and winter (midday averages of 0.51 and 0.23 μg [g DW]-1 h-1, respectively). In spring and summer, limonene was the monoterpene emitted at highest rate (midday averages of 5.27-6.69 μg [g DW]-1 h-1), whereas α-pinene was emitted the most in autumn and winter (midday averages of 0.31 μg [g DW]-1 h-1). The monoterpenes limonene, α-pinene and β-pinene represented about 75-95% of total detected monoterpenes. The total monoterpene emission rates represented about 0.04% of carbon fixed in autumn, 0.17% in winter, 0.84-2.51% in spring and 1.22-5.13% in summer. Significant correlations of total monoterpene emission with temperature were found when considering either summer emission or the emission over the entire year, whereas significant correlations with net photosynthetic rates were only found when considering summer season. Among individual terpenes, the most volatile, α-pinene and β-pinene, were more correlated with temperature than with net photosynthetic rates whereas the less volatile limonene was more correlated with net photosynthetic rate. Thus, under field conditions it seems that dependency of monoterpene emission on photosynthetic rate or temperature is partly related with volatility of the compounds. Influences of seasonality, temperature, photosynthetic rates and volatility should be considered in inventories and models of emission rates in Mediterranean ecosystems.