An 8·6 mm microwave bridge has been used to determine the complex permittivity of water over the temperature range 1-60°C to within a standard error of 0·25 in both ε' and ε". These results indicate that the high-frequency dielectric constant ε_[infinity] varies with temperature from 4·9 ± 0·3 at 1°C to 3·1 ± 0·8 at 60°C, which is a new proposal for water. As with previous investigations, it is difficult to discriminate between Debye behaviour and a small distribution of relaxation times, but the present measurements conform to the latter type of process below room temperature with increasing approximation to a single relaxation time as 60°C is approached. Values of the relaxation wavelength λ_s, with an error of ± 1-2%, are also given and are slightly lower than those published hitherto.