Real-time measurement of fine particles in stack emission gases is an important component in continuous environmental monitoring of PM10 and PM2.5. We developed a porous tube dilutor (using both hot and cold dilution) to measure fine, but not condensable, particles in highly humid gas emissions and compared our device to a commercial ejector-type dilutor. Particle size distributions were measured in emissions from a diesel engine and a coal-fired boiler. The porous tube dilutor successfully measured particles in accumulation mode, including relatively large particles over 3 µm in diameter (without nuclei), whereas the ejector dilutor detected some condensable particles but could not detect large particles. The porous tube dilutor successfully removed condensed water droplets generated by a humidifier in a 30-m3 chamber.