Peritubular dentin (PTD) is a relatively dense mineralized tissue surrounding tooth dentin tubules, whose composition and mode of formation are still unclear. Fourier transform infrared microspectroscopic studies of the organic and mineral components of the highly developed horse PTD indicate that the peritubular matrix is less abundant than the intertubular matrix but is also mainly composed of collagen, which is more hydrated. These data suggest that most of the crystals are located outside the collagen fibrils and probably not associated with protein components. The crystals in PTD have nearly the same crystallinity as those in intertubular spaces, showing comparable amounts of carbonate ions, although some PO4 groups have different nonapatitic environments. Horse PTD composition is very similar to that of ITD, with collagen as the main protein component and carbonated apatite as the mineral fraction but the different proportions of these constituents and the greater water content in PTD suggest a different organization.