The House of Marcus Lucretius preserves the remains of paintings of the so-called Pompeian second, third and fourth styles. In this work, samples from the second style painting (architectural style AD 40–80), buried in the ground and not exposed to open air, recovered from recent excavations (2004–2006), were analyzed. Moreover, wall paintings and wall fragments from the fourth style (baroque ornate style, before Vesuvius eruption), excavated about 150 years ago and exposed since then to outdoors, were also analyzed. Raman spectroscopy was used to characterize the original composition and decay products in the mortars and pigment layers. These spectroscopic results were complemented with quantitative analysis (ionic chromatography) of soluble salts and chemometric and chemical equilibrium calculations. Probable decay pathways are proposed to explain the formation of some decay products of red pigments and for the original components of the mortars. Moreover, a final diagnosis as well as comparison of the conservation state of the mortars and pigments exposed to two different kinds of environments (outdoors and under a burial) is discussed emphasizing the importance of SOx impacts on the open air artworks. Copyright © 2010 John Wiley & Sons, Ltd.