An exhaust gas recirculation (EGR) system for an engine and a method of operating that system is disclosed. The system has a conduit arrangement for conducting exhaust gas from an exhaust side of the engine to an intake side of the engine, a valve arrangement configured for controlling the amount of exhaust gas to be recirculated and a conduit arrangement for providing intake air to the intake side of the engine. A sensor arrangement is provided and is configured to sense at least one parameter indicative of the humidity of the recirculated exhaust gas and the intake air at the intake side of the engine. A control arrangement is configured to receive a signal from the first sensor arrangement and further is configured to control the valve arrangement in response to a determination by the control arrangement that the first parameter is outside a desired range for low-NOx emission during a subsequent combustion period.