Leaf isoprene emission rates (Fiso) were studied in 2-year old trees of live oak (Quercus virginiana Mill.) during two drying–rewatering cycles. During the first drying–rewatering cycle, photosynthesis (A) and stomatal conductance (gs) decreased by 92%(±7%) and 91%(±8%), respectively, while Fiso remained essentially constant for 8 days of treatment. After 12 days under severe drought conditions, Fiso was reduced by 64%(±6%). Similar values were found during the second drying–rewatering cycle. During the recovery phase of both cycles, Fiso recovered more quickly than A and gs. The lower drought sensitivity of Fiso compared with that of A resulted in a higher percentage of fixed C lost as isoprene (Ciso/CA) as plants became more stressed, reaching peaks of 50% when A was almost zero. Fiso showed a strong negative linear relationship with pre-dawn leaf water potential (ψPD) that could be a useful parameter to include in isoprene emission models to account for effects of drought stress on leaf Fiso.