The EU regulation 1107/2009 introduces the substitution principle for active substances in plant protection products that meet certain human or environmental hazard criteria. For products containing such candidates for substitution, their risk must be compared to alternative products during authorisation procedure. This report presents an approach for comparative risk assessment based on a profile of all different endpoints currently used in environmental risk assessment of plant protection products. It further suggests decision criteria for identifying significant differences in environmental risk, which would justify the substitution of a candidate product by a less critical alternative.