The solubility (m,at) of cesium chloride (CsCl) in water was measured in the pressure and temperature ranges, 0.10-400 MPa and 273-313 K, respectively. The following function was fitted to the results: m(sat) (mol kg(-1)) = 11.000 - 8.80 x 10(-3) p + 3.3 x 10(-6) p(2) + (0.0675 - 4.167 x 10(-5) p) (T - 293.15) - 0.81 X 10(-4) (T - 293.15)(2). The standard deviation is 0.022 mol kg(-1). This level of precision in solubility is almost an order of magnitude better than the literature data for the same system at the atmospheric pressure. This improvement in reproducibility is due in part to a better design of the inside of the pressure vessel used for the present study. The pressure and the temperature coefficients of the solubility were thermodynamically estimated using relevant data of the solute and the solution in literature. To supplement the volume data, we also measured the density of aqueous solution of cesium chloride at high concentrations. The coefficients thus calculated were the same within experimental error with those derived from our data, the equation shown above. It proves our solubility measurement under high pressure reliable.