The absorption of photons from a high-intensity beam of laser radiation perturbs the concentration difference between the two states coupled by the incident radiation. This perturbation is investigated by a second beam using well-known pump and probe methods. By using picosecond mode-locked lasers and high-frequency modulation methods the sensitivity limit of the technique developed, termed concentration-modulated absorption spectroscopy, COMAS, is extended dramatically and compares most favourably with fluorescence excitation spectroscopy. The quantitative treatment of the theory shows that the gain created on the probe laser can be related formally to the transmittance of the sample, the relationship providing a direct measure of the concentration of the absorber. This COMAS approach to concentration measurement dispenses with the need for calibration using a known concentration of the chromophore or an accurate knowledge of the appropriate absorption coefficient. Accordingly, it is most suitable for investigating species concentrations in mixtures in the gaseous and in the liquid states. Experimental results using lithium in a hydrocarbon flame and thionin (Lauth's violet) in aqueous solution are reported. Some of the wide ranging applications of this technique are discussed.