Measurements of thermophysical properties (vapor pressure, density, and viscosity) of the water + lithium bromide + sodium formate system (LiBr:CHO2Na =2:1 by mass ratio) and the water + lithium bromide + potassium formate system (LiBr:CHO2K =2:1 by mass ratio) were measured. The system, a possible new working fluid for absorption heat pump, consists of absorbent (LiBr + CHO2Na) or (LiBr + CHO2K) and refrigerant (H2O). The vapor pressures were measured in the ranges of temperature and absorbent concentration from 293.15 K to 343.15 K and from 20.0 mass % to 60 mass %. The experimental data were correlated with an Antoine-type equation. Densities and viscosities were measured in the same range of temperature and absorbent concentration that vapor pressure. Regression equations for densities and viscosities were obtained with a minimum mean-square-error criterion.