A dual ion beam system is used to deposit thin films in the presence of ion bombardment. This technique provides a measurement of sputter yields of elements in alloy films. Material is deposited by ion beam sputtering using a high current focused beam, while a second ion beam bombards the growing film. The film composition is modified by preferential resputtering of the high sputter yield component, analogous to the effect of substrate bias in diode sputtering. The advantage of the ion-beam technique is that ion energies, currents and angles may be directly measured and controlled. We have studied the sputter yield ratio of Gd to Co in binary Gd–Co films deposited with simultaneous ion bombardment by an argon beam of typically 500 eV ion energy. Gd is preferentially resputtered with a yield ratio which increases from 2 to 6 with increasing Co concentration in the film. This dependence of yield ratio on film composition is consistent with a model in which surface binding energy dominates the sputter yields.