Environmental Context. Many plant species biosynthesize and emit the volatile hydrocarbon isoprene. Once in the atmosphere, isoprene is susceptible to a range of reactions involving potentially hundred of products and intermediate compounds. The products of these reactions in turn may pose a risk to human and plant health and impact the climate through the generation of acids, ozone, and atmospheric aerosols. Abstract. The atmospheric oxidation mechanism of isoprene initiated by OH, O3, NO3, and Cl, which incorporates the most recent laboratory and theoretical studies, is described. A box model intercomparison between the new mechanism and previous available isoprene oxidation mechanisms has been performed. Ozone and OH concentrations are compared with predictions by the previous mechanisms in high and low NOx scenarios. The O3 and OH sensitivities to the chlorineisoprene reactions have also been investigated by comparing the box model results with and without the chlorineisoprene reactions, showing that the ozone production rate and OH concentrations are slightly impacted. The new mechanism facilitates more accurate modelling of isoprene photochemistry in the atmosphere.