The microstructure of Co/Cu multilayers deposited by low energy ion-assisted deposition was investigated. The samples studied were grown by unbalanced magnetron sputtering with and without an applied d.c. substrate bias of -50 V. Specular and off-specular X-ray reflectivity measurements were performed on the samples and revealed the presence of roughness at the interfaces that was partially correlated throughout the film. The effect of applying a -50 V bias was to suppress the correlation of the lower frequency roughness and to slightly reduce the bilayer period of the multilayer. The differences between the samples are discussed in terms of possible ion bombardment induced smoothing of the layers and densification of the microstructure.