A new photochemical method is presented for the artificial ageing of the triterpenoid resins dammar and mastic, which are commonly used as painting varnishes. Varnish samples in solution are exposed to the radiation of a fluorescent tube device. The solvents dichloromethane (DCM) and acetone, and the photosensitisers Merocyanine 540 (MC540) and FotoFenton 2 (FF2) dissolved in acetonitrile (ACN) or DCM were found to generate reactive species, which induce similar oxidation and cross-linking processes in the triterpenoid samples as found in varnish films on painting. This method provided information on the oxidation mechanisms of the model compounds hydroxydammarenone (3) and olealonic acid (8). Furthermore, evidence was found that the formation of high molecular weight material is important for the yellowing behaviour of triterpenoid varnishes. This method of solvent ageing can be successfully used for the preparation of cross-linked fractions of aged dammar and mastic varnishes.