Gear lubrication is critically important to maintaining the integrity of operating gears, the lubricant also protects asperity contact at the gear mesh thereby protecting the gears from a deterioration process and surface failures. In this paper, the investigation was centred on the application of the acoustic emissions (AE) technology for monitoring the influence of oil film thickness variation on gear contact and characterising the oil lubrication regimes in helical gear mesh. This investigation employed a back-to-back gearbox test-rig with oil-bath lubrication. The results have demonstrated a clear relationship between AE activity, operating temperature and specific film thickness. The findings encourage the use of AE techniques to detect and quantify the lubrication regimes during gear meshing.