Dielectric relaxation measurements in the frequency range 10 MHz–10 GHz have been carried out in tert-butyl alcohol–water mixtures with various concentrations over the temperature range 273–313 K using a time-domain reflectometry (TDR) method. The bilinear calibration method as suggested by Cole has been used to correct the permittivity spectra. The corrected spectra could be fitted with a single relaxation time with a small amount of Cole–Davidson behaviour. Deviations from ideal mixing behaviour in the permittivity parameter (ε_o–ε_∞) and relaxation time (τ) suggested the formation of a polymeric structure in tert-butyl alcohol–water mixtures. The dielectric relaxation behaviour showed the same structural changes as observed in ultrasonic relaxation. However, the maxima in excess permittivity and excess relaxation time occurred at different positions. This could not be explained by a simple model of the polymeric structure.