This work investigated the structural and chemical changes of plastics undergoing accelerated ageing upon irradiation that simulated the ultraviolet component of solar radiation for a five-year period. The plastics selected were polyethylene (PE) and polypropylene (PP) microplastic debris sampled from a sandy beach as well as pure pellets and fragments of objects made of the same polymers. We recorded Fourier Transform Infrared (FTIR) spectra at prescribed intervals during the irradiation procedure. The spectra were used to study the evolution of the absorption peaks usually associated with the environmental ageing of polyolefins, namely the peaks of hydroxyl and carbonyl stretching, the peaks relating to the presence of double bonds, and those associated with the crystallinity of PE and the tacticity of PP. The results showed that none of the usual degradation indexes followed a clear trend with increasing exposure and that the evolution of absorption peaks was not consistent among different fragments. We used the Orthogonal Partial Least Squares Discriminant Analysis (OPLS-DA) method to process the whole information contained in the FTIR spectra in response to the chemical changes occurring during photochemical ageing. The results showed that the FTIR spectra contained sufficient information to cluster samples according to the irradiation received. Variable Importance of the Projection (VIP) analyses showed that the information for discriminating among different exposures was mainly contained in the absorption peaks corresponding to the hydroxyl and carbonyl stretching absorptions. The chemometric models had large determination coefficients, despite the large number of variables involved and could be applied to assess the environmental fate of plastics under environmental stressors.