Several studies have used Fourier Transform Infrared Spectroscopy (FTIR) to assess chemical and structural changes caused by diagenesis in archaeological human bone, whereas other factors such as individual’s biological profile (sex and age) or the type of bone have seldom been considered. In this study transmission FTIR was applied to 51 bone samples from 19 post-Roman individuals of A Lanzada necropolis (NW Spain). Mid-infrared (MIR) indices (IRSF, MMI, C/P, C/C, Am/P, BPI, API, AmI/AmII) were also calculated and principal component analysis (PCA) was used to explore peak ratios and differences across the whole spectrum. PCA components showed correlation to the C/P and Am/P indices, as well as differences in the Amide III absorbance trends versus Amide A, B, I and II. Signals related to soil material (silicates and aliphatic organic matter) were also revealed by the PCA in some samples. No significant differences in bone composition per sex were found, but cranial carbonate content was significantly higher in non-adults than in adults, and ribs presented a higher amide-to-phosphate ratio (Am/P) than femora and crania. Ribs showed the most altered bioapatite, in agreement with a previous study based on the elemental composition of the samples analysed here. Bioapatite alteration may be responsible for the higher amide content relative to phosphate (i.e., preferential preservation of collagen) in ribs. Thus, caution is advisable when using the Am/P index to assess collagen preservation.