Argon clusters (Ar-30-400) produced in a supersonic expansion were doped with water using the pickup method and subsequently electron impact ionized. Charged fragments were detected by a triple-quadrupole mass spectrometer. The intensities of the water-containing fragment peaks (ArnH2O+ n = 4-7) versus water pressure followed Poisson distributions, from which capture cross sections were derived. These cross sections corresponded to cluster sizes in agreement with published results for our range of stagnation conditions. The measurement of water capture cross sections has been shown to provide a convenient means of determining van der Waals cluster mean sizes.