This study reports the results of an experimental work carried out with an elastically mounted rigid cylinder exposed to an oscillating flow. The cylinder is free to vibrate in the transverse direction. The effect of Reynolds number on the vibrations has been investigated up to Reynolds numbers of about 4 × 105 . The Keulegan-Carpenter number of the experiments covers the range 10 ≤ KC ≤ 100. The range of the reduced velocity is from 0 to approximately 16 in most of the cases. The tests have been conducted with both smooth and rough cylinders. The present study demonstrates that the effect of Re number on vibrations can be substantial, depending on the surface roughness of the cylinder. The effect is most pronounced when the cylinder is smooth.