The mechanisms of red lead degradation were studied in a medieval Portuguese codex, Lorvao Apocalypse (11189), by Raman microscopy (mu-Raman) and micro-X-ray diffraction (mu-XRD). The range of pigments found for the illuminations is mainly limited to vermilion, orpiment and red lead. Micro-Fourier transform infrared spectroscopy (mu-FTIR) determined that the pigments were applied in a proteinaceous binding medium. In the red and orange colours, arsenic (As) was determined, by micro-energy dispersive X-ray fluorescence (mu-EDXRF), to be ranging 1-4% (wt %). For those colours, lead white and calcium carbonate were found as fillers whereas orpiment was applied as a pure pigment. Raman microscopy identified, unequivocally, the degradation product of red lead as galena [lead (II) sulphide, PbS]. To determine the main factors affecting red lead degradation, a set of accelerating ageing experiments was designed to assess the influence of extenders and of the two other pigments, vermilion and orpiment. The experiments were followed by mu-Raman, mu-EDXRF and XRD. Raman microscopy results for the simulation of degradation of red lead, in the presence of orpiment, are in agreement to what was found in the Lorvao Apocalypse, galena being the main degradation product; also in common is a Raman band at ca. 810 cm(-1), which was attributed to a lead arsenate compound. It was concluded that in Lorvao Apocalypse, the degradation of red lead was a result of its reaction with orpiment. Copyright (C) 2009 John Wiley & Sons, Ltd.