This paper reports on real-time mass concentration measurements using the technique of photoacoustic spectroscopy. The measurements were made using a differential photoacoustic cell in which infrared absorption by diesel particulates at the 10.59 μm wavelength was measured with a 2.2 watt CO2 laser. Infrared absorption of the diesel particulates at 10.59 μm wavelength was treated as Rayieigh absorption since the mean mass diameter is around 0.1 μm and small enough to neglect the size effects. A dual cell method allows the separation of the particulate absorption signal from that of interfering gases present in diesel engine exhaust. In practice the sample gases from the dilution tunnel were introduced into two cells, one with and one without a particulate filter. The signals were calibrated by simultaneous measurement using the diesel particulate filter method of the U.S. EPA.The sensitivity of each PAS cell was found to depend upon the flow rate of the sample, the pressure in the cell, and the modulation frequency of the incoming light source. It was adjusted to be 2.0 mV/mWcm−1 at the longitudinally resonant frequency of 1.69 kHz where background acoustic noise is negligibly small. The results indicate that the PAS signal is linearly proportional to the mass concentration of the particulates in the range of 2.0 ~ 140 mg/m3 within an accuracy of 15%. The response time of the measurement is less than 5 seconds.