In analytical chemistry, the molar absorptivity or molar extinction coefficient ε of a chemical species at a given wavelength is a measure of how strongly the species absorbs light at that wavelength. It is an intrinsic property of the species; the actual absorbance of a sample is dependent on its thickness L and the concentration c of the species.

When there is more than one absorbing species in a solution, the absorbance is the sum of the absorbances for each individual species. The absorbance at a given wavelength of a mixture of species X, Y, ... is given by

,

where A is the absorbance of the mixture. The composition of a mixture of two or more compounds can be found by using two or more wavelengths and by knowing ε at each wavelength for each compound. These wavelengths are usually chosen as the wavelengths of maximum absorption (absorbance maxima) for the individual components. For N components with concentrations c_i (i=1,...,N) and N wavelengths λ_i, absorbances A_i are obtained:

.

This set of equations can be solved for the concentrations c_i, provided that none of the wavelengths is an isosbestic point for any pair of species. An isosbestic point of two species is a wavelength where the two species have equal extinction coefficients.