The interfacial structure and fluctuations of a L-alpha-dipalmytoilphosphatidylcholine monolayer at the hexadecane/water interface were investigated using grazing-incidence surface scattering of X-ray synchrotron radiation. The interfacial structure of the densely packed monolayer is consistent with that of a similar monolayer at the air/water interface. The fluctuations are shown to be limited by interfacial tension on length-scales down to in-plane wavelengths of a few tens of nanometers. This result at the alkane/water interface demonstrates the power of the X-ray grazing-incidence technique applied to the study of liquid-liquid interfaces.