The extraordinary capacity of isolated mitochondria to accumulate Ca²⁺ has been established for more than 40 years. The distinct kinetics of the independent uptake and efflux pathways accounts for the dual functionality of the transport process to either modulate matrix free Ca²⁺ concentrations or to act as temporary stores of large amounts of Ca²⁺ in the presence of phosphate. One puzzle has been the nature of the matrix calcium phosphate complex, since matrix free Ca²⁺ seems to be buffered in the region of 1–5 μM in the presence of phosphate while millimolar Ca²⁺ remains soluble in in vitro media. The key seems to be the elevated matrix pH and the third-power relationship of the PO₄ ³⁻ concentration with pH. Taking this into account we may now finally have a model that explains the major features of physiological mitochondrial Ca²⁺ transport.