Osteoarthritis naturally occurs in young adult female cynomolgus monkeys and it closely resembles the human disease. In the knee joint, it is characterized by the breakdown of the articular cartilage and marked thickening of the adjacent subchondral bone. There is a strong correlation between the severity of the osteoarthritic lesions and the thickness of the subchondral bone, but it is unclear why this is the case. We question whether the newly-deposited subchondral bone Hhas a different chemical structure/composition compared to the older Hbone, leading to the development of osteoarthritis. Presently, we are Husing infrared micro-spectroscopy to study the chemical composition of Hthe subchondral bone as a function of subchondral bone thickness. We Hare able to map the subchondral bone from the articular cartilage H(older bone) to the marrow space (newer bone) and compare their Hchemical compositions. Bone proteins, primarily collagen, absorb Hinfrared light in the 1450 - 1650 cm-1 region. An intense, broad band Hfrom 900 - 1200 cm-1 and a double-peaked band from 500 - 650 cm-1 are attributed to bone mineral, i.e. phosphate ions in hydroxyapatite. A small absorption centered at 875 cm-1 arises from carbonate-substituted hydroxyapatite. These bands are sensitive to mineral content (i.e. carbonate, phosphate, acid phosphate), mineral crystallinity, and the content/nature of the organic matrix. They are Hcurve-fit and/or integrated and compared as a function of (1) state of Hthe bone, i.e. thickness of the subchondral plate, and (2) age of Hbone, i.e. position in the subchondral plate.