¹H transverse nuclear magnetic relaxation times (T₂) and self- diffusion coefficients (SDCs) of water were measured in isolated proteoglycan aggregates from pig articular cartilage. The influence of varying osmotic pressure, as well as of different calcium concentrations, on the samples was investigated. Due to a structural transition of the proteoglycans that results from changed electrostatic interactions at higher calcium concentrations, an additional fraction of water protons is observable. These protons are characterized by a very long T₂ value and low, restricted diffusion. Additionally, electron microscopic elemental analyses and XFA investigations were performed to estimate the amount of calcium taken up by the proteoglycans. A model for the calcium-mediated structural transition of the cartilage proteoglycans is proposed that explains the experimental results. The investigations suggest the ability of proteoglycans to act as a calcium-concentrating agent and suggest their important role in the calcification process of articular cartilage.