Clusters generated by injecting vapor from water-methanol liquid mixtures of varying compositions through a pulsed valve, have been analyzed using a time-of-flight mass spectrometer. Neat clusters of water and methanol as well as mixed species have been observed, the relative populations varying with composition. The mole fractions in the vapor deviate significantly from the values calculated assuming ideality of the liquid except at water mole fraction of 0.67, the composition around which the excess enthalpy is minimum. An anomaly observed in the cluster populations at this composition is related to an extended hydrogen bonded interaction of molecules in the liquid. © 2001 Elsevier Science B.V.