Abstract: An apparatus and method for maintaining a constant dilution ratio for a dilution probe used in a continuous emission monitoring system. The apparatus includes a regulator for regulating the flow of dilution gas to the probe, a mechanism for measuring changes in gas density of the emission gas and a mechanism for determining an adjusted flow rate for the dilution gas to the dilution probe based on the measured changes in gas density to control the dilution ratio. The method involves monitoring the gas density of the emission gas for a change in gas density. Upon the occurrence of a change in gas density, an adjusted flow rate of the dilution gas as required to control the dilution ratio is determined, and then the flow of dilution gas to the probe is adjusted to the adjusted flow rate.