The excess enthalpies, H(E), for mixtures of the nonionic amphiphile C4H9OC2H4OH + water were measured by isothermal flow calorimetry at atmospheric pressure and 10 temperatures over the temperature range T - T(lcst) = -0.1 to +32 K. (T(lcst) is the lower critical solution temperature.) The H(E) data are reported and compared to literature values. The phase boundaries between the single- and two-phase regions, determined from the H(E) data, are also reported and compared to data from the phase volume method and from the literature.