Organosulphur compounds have been long known to be effective antiwear (AW) and extreme pressure (EP) additives in a wide range of metalworking applications. Several interpretations to explain the properties of these compounds have been proposed, but the mechanisms and surface reaction products are still largely unknown. To understand the tribochemistry of these additives, especially thiols and polysulphides, a new device called 'environmentally controlled tribometer' was developed. It permits to simulate the action of the additives using gases having the same chemical function. After friction experiments, surface analyses were carried out on the tribofilm in order to understand the tribochemical mechanism. The evolution of the friction coefficient versus partial pressure of gaseous additive and sample temperature allowed clear differentiation of organosulphur compounds reactivity. The studies showed that the lubricating properties of the additives depend both on the quantity of sulphur and on the carbon chain length. The results for thiols series showed an increasing of EP and AW efficiency with increasing carbon chain length. The comparison between the liquid phase lubrication and the gas phase lubrication results validates this methodology.