An experimental study of the vortex-induced-vibration of a flexibly mounted rigid square cylinder in a uniform airflow is presented. For this high mass ratio configuration, transverse oscillations are measured in detail for reduced velocities ranging from 5 to 20. In the lock-in region and starting from rest, the cylinder motion exhibits a transient regime followed by a limit cycle oscillation regime. New experimental data are provided in term of amplitudes and frequencies of the limit cycle oscillations. The transient behaviour is also characterized by measuring the growth rate of the oscillations amplitude. © 2009 Académie des sciences.