Using the resonant frequency of a coaxial cavity excited in TEM modes at frequencies between 50 and 480 mgcy./sec., the dielectric constant of water was determined at 5°intervals between 0 and 70°, and at 100 bar intervals between 1 and 1000 bars. The results are expressed as a set of isothermal and isobaris equations whose parameters were evaluated by the method of least squares. The dielectric constant of water and its temperature and pressure derivatives at 1 atmosphere are tabulated at 5°intervals from 0 to 70°.