An account is given of measurements (carried out in March 1947) of the reflection coefficient of water at a wavelength of 8.7 mm. over a range of angles of incidence. The method employed is to measure the relative field strengths of the direct waves and waves reflected from a trough of water using free-space propagation and high-gain aerials. The following electrical constants of water have been computed from the measured results (water temperature 11.1° C.): refractive index 4.40±0.24, dielectric constant 10.86±2.21, absorption coefficient 2.91±0.06, conductivity/frequency 12.82±0.42, Brewster angle 79°.