The thermal-ageing of a series of commercial acrylic/methacrylic resins, homo- and copolymers which are extensively used as stone protectives, has been investigated under conditions of constant temperatures at 110, 135 and 150 degrees C. Structural and molecular changes induced by the isothermal treatments in a forced-air circulation oven were followed by infrared and UV-Vis spectroscopy, and size exclusion chromatography (SEC), respectively. The stability of the resins appeared to be controlled by the reactivity of alkyl side groups, whose oxidative decomposition is favoured in the case of long ester groups, like the isobutyl and butyl ones. At the same time, the polymers containing long eater groups undergo fast and extensive cross-linking, together with loss of short chain fragments. In the acrylic/methacrylic resins where all or the majority of the alkyl side groups are short, chain scissions prevail over cross-linking and no insoluble fractions were formed.