Formaldehyde (FA) is an ubiquitous gas in the atmosphere which reaches notable concentrations in polluted areas and can have great impact on human health. We studied FA exchange between air and two widespread Mediterranean tree species, Quercus ilex and Pinus halepensis. Experiments were conducted at the leaf level under laboratory conditions using air from outside the building. In both plant species FA exchange was mainly determined by the atmospheric mixing ratios, with a compensation point calculated around 20 ppbv. Higher values led to uptake and lower values to emission. The second factor that regulated FA exchange was stomatal conductance. FA exchange followed a diurnal cycle with the greatest exchange when stomatal conductance was at maximum. Such stomatal control is consistent with previous studies and is probably due to the high water solubility of FA, resulting in stomatal transpiration being its main exchange pathway. We also observed this relationship between stomatal conductance and FA exchange under conditions of drought and posterior rewatering, in which changes in stomatal conductance were paralleled by changes in FA exchange. Under projected future conditions of enhanced aridity in the Mediterranean, drought-driven limitations of FA exchange may be more relevant.