Most of the studies investigating the diagenetic trajectory of fossil bones focus on open-air sites and very little work have been published in confined environments such as catacombs. While the stable thermal history of catacombs should favor bone preservation, the accumulation of corpses over a short period of time could favor their destruction. The aim of this study is to describe the diagenetic trajectory of 128 human bone samples coming from six different burial chambers of the catacomb of Sts Peter and Marcellinus (SSPM, Rome, Italy). A multi-proxy approach was undertaken to provide an assessment of the molecular preservation as well as a direct record of the isotopic composition itself. Collagen yield, carbon and nitrogen abundances, C:N ratio, FT-IR based collagen and carbonate contents and crystallinity index, radiocarbon dating and stable isotope analysis of bone collagen and carbonate indicate that both the mineral and the organic fractions are impacted by diagenesis to various degrees, and that bones originating from the small burial chambers are more affected than those coming from the large ones. While some of the bones were strongly recrystallized, the impact of bone diagenesis on the stable isotope values of bone carbonate was limited. Comparison with contemporary sites from the Latium showed that conditions prevailing in catacombs seem overall to favor, rather than disadvantage bone preservation.