An experimental investigation of the cross-flow past a pair of staggered circular cylinders, with the upstream cylinder subject to forced harmonic oscillation transverse to the flow direction, is presented. Flow- visualization and hot-film wake spectra are reported for Reynolds numbers, based on upstream velocity and cylinder diameter in the range 1440 ?T Re ?T 1680. The longitudinal separation between cylinder centres is L/D = 2.0, with a transverse separation (for the mean position of the upstream cylinder) of T/D = 1.0. The oscillation of the upstream cylinder causes considerable modification of the flow patterns compared to what is obtained when the cylinder is fixed; depending on the frequency of oscillation, lock-in between the wake periodicity and the cylinder motion occurs. Indeed, two distinct types of wake synchronization are identified, where either the shear-layer shedding frequency or the frequency of the combined wake synchronize with the oscillation frequency.