Phase equilibria of the ZrO2-SmO1.5 system have been studied by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The compositions of phases in the tetragonal+fluorite, fluorite+pyrochlore, and fluorite+B-Sm2O3 two-phase fields have been determined for samples quenched from temperatures between 1400 degrees and 1700 degrees C. The heat content of the fluorite phase with 30 mol% SmO1.5 and of the pyrochlore phase with 50 mol% SmO1.5 has been measured in the temperature range 200 degrees-1400 degrees C using high-temperature drop calorimetry. The transition between pyrochlore and fluorite phases is clearly first order in the SmO1.5-rich region, while no fluorite+pyrochlore two-phase region has been detected for the samples with ZrO2 excess. Based on the obtained experimental results and literature data, the phase diagram and thermodynamic properties were optimized using the CALPHAD approach.