At high concentrations of an infrared absorbing gas, the optical absorption length for the incident radiation can become small compared with the thermal diffusion length in the gas. A description of the optoacoustic effect consequently must include thermal conduction of heat to the cell window since sound generation takes place in a thin layer of gas adjacent to the entrance window. A piston model is given here for the optoacoustic effect generated in a highly absorbing gas; in addition, experiments are reported that show new phase and amplitude dependences of the acoustic signal on gas concentrations, and modulation frequency.