An inertial filtration dilution probe for sampling stack gas for analysis. The probe is based on a body having a central chamber passing from a first end of the body to a second end. One end of the central chamber is connected to a sampling tube to be inserted into a stack and the opposite end of the body has a port which is adapted to be selectively connected to a source of calibration gas or blowback gas. A filter divides the central chamber into an inlet portion and an outlet portion, with the sampling tube and port connected to the inlet portion. The probe further comprises a dilution eductor joined to the body, for mixing filtered stack gas with dilution gas and comprising an inlet for dilution gas, an outlet for diluted gas and a vacuum cavity therebetween. A sample passage within the body connects the outlet portion of the chamber to the vacuum cavity and a heating means encloses the body and eductor. The probe allows more accurate calibration of the gas analysis, since the calibration gas follows in the same path as the stack gas.