Ambient gas-phase peroxide concentrations were measured at a forest site in Portugal during FIELDVOC, June 1994. Analysis was by HPLC with specific hydroperoxide detection. Hydrogen peroxide (H2O2) and methyl hydroperoxide (MHP) were quantified. H2O2 concentrations ranged from below the limit of detection up to 0.63 ppbv. MHP concentrations reached up to 0.1 ppbv. Both peroxides displayed diurnal cycles with higher concentrations during the daytime than during the night. Approximate backward air mass trajectories were determined from surface pressure charts for the periods sampled. The site was found to experience both Atlantic air and continental air. H2O2 concentrations were found not to be significantly different in air masses of different origins. MHP concentrations were found to be consistently higher in air of continental origin.