The surface pressure-area isotherms of monolayers of several phospholipids are markedly altered by the presence of phosphotungstate ions in the aqueous subphase at low values of pH (Gorwyn, D.; Barnes, G. T. Langmuir 1990, 6, 222). These changes have now been studied by grazing incidence synchrotron X-ray diffraction. Where the monolayer was in a condensed monolayer state the 2 theta scans revealed a single peak consistent with hexagonal packing of the acyl chains despite the diacyl structure of the phospholipids. Addition of phosphotungstate to the subphase at pH 2 caused this peak to disappear, suggesting a disruption of the regular chain packing, and examination of the concomitant surface pressure-area isotherms showed that there had been a transition from a condensed to an expanded phase.