Red lead was frequently employed as a pigment in paintings, especially in medieval wall-paintings. In unfavourable environmental conditions, a strong blackening of the pictorial layer occurs due to the instability of the pigment. In order to establish the causes and processes of this alteration, two kinds of aged samples were studied. Naturally aged samples were taken from an experimental wall-painting, executed in 1977 in a sheltered outer environment. Moreover, accelerated ageing tests were led on samples with an analogous preparation, using a climatic chamber. The samples were characterized using optical and electron microscopes (SEM and HRTEM), XRD, FTIR, and EDS. The naturally aged sample had a darkened surface, due to the formation of plattnerite (black PbO2) and gypsum (CaSO4.2H2O). However, the artificial ageing test did not induce a mineralogical nor chromatic change of the pigment. A redox reaction process in aqueous environment is proposed to explain the oxidation of the pigmet (Pb3O4+PbO) to plattnerite. -- SCMRE