The total vapour pressures of t-butyl alcohol (TBA) and water mixtures have been measured at 283.15, 288.15, 293.15, 298.15 and 303.15 K, respectively. Maxima and minima were observed in the vapour pressure versus concentration curves at the three lowest temperatures. The anomalous behaviour of the solution can be explained by the existence of a clathrate-like structure, which has been suggested by other authors.