PRICE II is a formal laboratory intercomparison of techniques to measure atmospheric concentrations of peroxy radicals (HO2 and RO2). It follows up the intercalibration exercise PRICE performed in 1994 as part of OCTA. The project joins the expertise of eight laboratories from four European and two North American countries. The main objectives of the project are: - To characterise and compare existing calibration methods for peroxy radicals. - To determine the relative response of the Chemical Amplifier for HO2 and different organic peroxy radicals (RO2). - To develop a common model of chemical amplifiers based on the laboratory results. - To define the optimum operating conditions for chemical amplifiers with respect to HO2 and RO2 detection.