A system is provided in an exhaust emission sampling system for controlling a flow controller in order to compensate for the effects of changing water vapor content in a diluted sample having a predetermined dilution ratio. A water measuring device such as a relative humidity sensor generates a water vapor signal based on the amount of water vapor in the diluted sample. The water vapor signal is then processed within a control unit to obtain a value for the amount of water in the diluted sample. A control signal is generated by the control unit based on the amount of water vapor, flow rate of the exhaust emissions, and also, on the type of fuel used in the combustion process to control the flow controller which is, preferably, a mass flow controller. A second system is provided in an exhaust emission analyzing system for determining a final amount of pollutants also compensated for the effects of changing water vapor content in the exhaust emissions. The control unit not only processes the water vapor signal from the sensor to obtain a value for the amount of water vapor in the diluted sample as in the first embodiment, but also generates a correction factor based on the value. The correction factor is then utilized by a computer to calculate a final amount of the pollutants based on the correction factor and a preliminary amount of pollutants determined by analyzers of the exhaust emission analyzing system.