A method to determine large optical absorption coefficients of liquids from the amplitude ratio of acoustic waves generated under rigid‐ and free‐boundary conditions is presented. This has been examined both experimentally and theoretically, taking into account the attenuation of the pulse amplitude ratio with the increased acoustic path length due to attenuation effects. Although the proposed method was tested in a narrow wavelength interval and close to the limit of precision of the experimental arrangement, the results show quantitative agreement with the literature values.