In order to check the pre‐harvest intervals established in Portugal for olive trees, residue dissipation studies have been carried out for the organophosphorus insecticides diazinon, dimethoate, methidathion, parathion and phosphamidon. Simul‐taneously with the analyses of the olives, olive oil produced in the laboratory by a mechanical procedure of pressing the olives was also analysed. The analyses were car‐ ried out by gas‐liquid chromatography. Residues found in the fresh olives, sampled at the end of pre‐harvest intervals in force for each of the insecticides, were accept able. However, for diazinon and parathion, unacceptable high residue levels were found in the olive oil. The concentration of methidathion residues in the olive oil was also observed. For dimethoate and phosphamidon, no detectable residues were found in the olive oil, even when the olives were sampled one day after spraying.