A comprehensive model is developed to calculate the maximum performance of a thermosyphon with a built-in crossover separator. Mechanisms limiting performance are considered to be a flow instability in a natural-circulation two-phase flow system at low reduced pressures (e.g., for Freon-11, p(r) < 0.126) and at low total mass flux and wave film spalling at moderate reduced pressures, respectively. Which limit becomes dominant depends on the operating conditions, as shown by the experimental data. In systematic experiments, various working fluids are used, viz., water, ethanol, Freon-11, and Freon-113. Operating temperature and liquid fill ratio are varied. The present model for maximum performance agrees well (within +/-15%) with experimental data.