The primary break-up of liquid jets in cross flows has been studied experimentally. An open-circuit wind tunnel was employed in which the airflow was generated by a centrifugal fan. The test section, positioned 3 m downstream of the fan, was made of clear acrylic resin to allow optical access and visualization. The working liquid used in the present experiment was an aero-engine lubrication oil, which was injected perpendicularly into the air flow, via a nozzle placed in the top wall of the test-section. The study of the primary break-up mechanisms of the jet involved three parameters, the oil viscosity, and the jet and air cross flow velocities, which were varied independently. Two different break-up regimes were observed and identified; arcade break-up and bag break-up. These were separated by a transition zone. Transverse and longitudinal (or stream-wise) penetrations of the jet before the liquid breakup were also measured. The correlation proposed by Wu et al. to predict the jet transverse penetration before the break-up of the liquid, as a function of the liquid/ airflow momentum-flux ratio, was found to be applicable only to liquids with low viscosity. An empirical extension to this equation has been produced.