Abstract: A cryogenic density and mass-flow measurement system incorporating a high-speed microprocessor-based cryogenic fluid density instrumentation that is based on the rigorous application of the molecular dielectric theory, using a dielectric susceptibility function in the application of the Clausinus-Mossotti formula, and the introduction of the quantitation of a new susceptibility parameter k which serves to bridge the gap between the theoretically rigorous molecular dielectric equation and the macroscopic dielectric equation. The operating principle is formulated on a differential dielectric measurement approach with a new algorithm which provides for automatic adjustments for polarizability and stray capacitance changes. High precision digital density measurement is achieved over a wide range of cryogenic fluid states ranging from supercritical through subcritical to the slush phase.