A photoacoustic method has been applied to the quantitative measurement of optical absorption at 10.6 &mgr;m in coated and uncoated alkali halide samples having a spatially varying optical absorption. Photoacoustic signals were measured as a function of the chopping frequency of a CO2laser source. The dependence of the signals on chopping frequency and an absolute electrical calibration of the cell were used to interpret the data. The results are in good agreement with laser calorimetry or infrared transmission measurements on the same samples. Signal fluctuations identified with thermal and source‐coherence effects are reported and discussed. The experiments reported here confirm the utility of the photoacoustic technique in the general investigation of optical properties of highly transparent materials.