Oil aeration can be a real problem in engine oil circuit. The involved lubrication power decrease and thermic properties changes can damage the engine. Furthermore, the increased compressibility is very dangerous for hydraulic systems like valve lash adjusters. A first step to control this aeration is to be able to measure it. Gammametry is often used but this measurement needs a very precise calibration and is quite complicated and dangerous. A new absolute measurement has been discovered, based on the difference of compressibility between air and oil. It is absolute because the measurement principle is independant of the conditions. The system does not need a new calibration at each new environment. It is valid for any gas-liquid mixture. From the (P,V) graph of a gas-liquid mixture compression, one can derive the gas-liquid volumetric ratio. The log-log graph (P/PO, 1-V/VO) of a mixture sample pressurization always shows an inflexion point. The y value of this inflexion point (1-V/VO) is equal to the volumetric gas ratio of the sample (before compression). This phenomenon is obvious on hydraulic curves (see Annexe I). To check it, we have proceeded to a theoretical demonstration and an experimental verification. The theoretical demonstration of this principle concludes that the principle is verified as long as the ration Po/alpha B is small. B is the oil bulk modulus, cc is the volumetric gas ratio and Po the sample initial pressure. Numerical simulations indicate that the pressurization must be regular and fast enough to avoid the problems coming from dissolution. They also indicate that small leakage could be profitable by decreasing the maximum pressure needed to meet the inflexion without modifying the measurement. The experimental verification consists in measuring the (P,V) graphs during the pressurization of different known mixture samples. The positions of the inflexion points of the log log graphs were compared with the known volumetric gas ratio, The correlation is very good and the deviations were inside the error range of the preparation of the mixture samples. Based on this principle, a system of oil aeration measurement has been developped:the SMAC (french name for Compressibility based oil Aeration Measurement System). The system is able to take samples from a given circuit by electrovalves. The sample is then regularly pressurized by a piston; pressure, piston displacement and temperature are stored along the time in a PC. The inflexion point is then determined, the gas ratio calculated and printed by a soft on the PC. The whole operation takes around three minutes, This apparatus has been used for two studies. The first one was dealing with hydraulic valve lash adhusters (HLA). The SMAC was measuring the gas ratio of oil in the circuit. The aim was to evaluate the influence of aeration on the HLA. The second study consists in defining a new oil aeration test method to measure the influence of silicone additives on aeration. Generally speaking, the SMAC principle can be used on almost any gas liquid mixture to measure the volumetric gas ration.