X-ray diffraction measurements have been made in real time during deposition of the rare earth Gd on the non-magnetic substrate Mo(110). For the clean surface at 300 K, the variation of the specularly reflected X-ray intensity with deposition time is consistent with the completion of a close-packed monolayer followed by increasingly disordered, multilayer, growth. Exposure of the Mo surface to 0.25 langmuir of oxygen prior to Gd deposition produced better defined intensity oscillations characteristic of improved layerwise growth. It is suggested that the oxygen creates nucleation sites which increases interlayer mass transport.