Photoacoustic absorption spectroscopy (PAS) was investigated as a method for on-line monitoring of carbon in fly ash from coal-fired boilers. PAS is based on the periodic heating of a gas when amplitude-modulated radiation is absorbed by the gas or by particles suspended in the gas. This periodic heating produces an acoustical wave that can be detected by a microphone. Because the PAS signal is based solely on the absorption of radiation (by carbon) and not scattering of radiation (by mineral matter), it has the potential for distinguishing unburned carbon from mineral matter suspended in the flue gas. Two radiation sources were studied: helium-neon laser and microwaves;The helium-neon source produces a large mass specific absorption coefficient for the carbon in the fly ash. Although the optical PAS system uses less than one forty thousandths of the power as the microwave system, it still has twice the sensitivity (D < 75 [mu]m). The helium-neon laser source is easy to work with and there is no pick-up of the electromagnetic field by the electronic equipment, but there is also a size dependence of the PAS signal to the carbon size distribution. This is a serious drawback because a carbon monitor should be able to perform without knowledge of the carbon particle size distribution;The microwave source produces a signal that is independent of the carbon particle size distribution, but the low mass specific absorption coefficient at 12.24 cm would require a microwave source producing 12000 watts to measure the carbon content of pulverized coal combustors. This is impractical. There are also problems with electronic equipment picking up the electromagnetic field generated by the magnetron leaking from the waveguide;An optimum source may be found in the spectrum between the helium-neon laser source ([lambda] = 632 [eta]m) and the microwave source ([lambda] = 12.24 cm). It may be possible to find a source that has all the positive characteristics with manageable drawbacks.