AbstractAt 25°C the complex (dielectric) permittivity of the system tert.‐butanol/water has been measured as a function of frequency v(3 MHz ≦ v ≦ 70 GHz) and mole fraction x (0 ≦ × ≦ 1). Different relaxation functions are fitted to the measured spectra. It is found that within the complete composition range an empirical function is appropriate, which is based on an unsymmetric continuous distribution of relaxation times. Other than expected on grounds of previous light scattering and ultrasonic absorption studies there are neither indications for the formation of microphases nor for special hydrophobic hydration effects. The parameters of the relaxation spectral function are discussed to yield information on the liquid structure and molecular motions of the mixtures. Comparison is made with further aqueous systems which had been measured over a wide composition range, in particular other alcohol/water mixtures but also aqueous solutions of nearly non‐dipolar quinoxaline and of dimethyl sulfoxide.