Abstract: Ozonolysis is a well known process involving reacting ozone with alkene compounds, for example in unsaturated vegetable oils or free fatty acids and esters thereof, to form ozonolysis products (e.g. ozonides). The invention concerns ozonolysis of unsaturated oils (e.g. unsaturated plant oils and/or unsaturated animal oils) in the presence of a participating co-reactant to form reaction products particularly suitable for use in the formation of resins.