The Molar absorption coefficient, molar extinction coefficient, or molar absorptivity, is a measurement of how strongly a chemical species absorbs light at a given wavelength. It is an intrinsic property of the species; the actual absorbance, A, of a sample is dependent on the pathlength l and the concentration c of the species via the Beer-Lambert law, A = εcl.

The SI units for ε are m²/mol, but in practice, they are usually taken as M^-1 cm^-1 or L mol^-1 cm^-1. In older literature, cm²/mole is used. These units look very different, but it is just a matter of expressing volume in cm³ or L.

Different disciplines have different conventions as to whether absorbance is Naperian or decadic, i.e. defined with respect to the transmission via natural or common lograthim. The molar absorption coefficient is usually decadic,^[1] but when ambiguity exists it is best to qualify it as such.

The molar extinction coefficient should not be confused with the different definition of "extinction coefficient" used more commonly in physics, namely the imaginary part of the complex index of refraction (which is unitless). In fact, they have a straightforward but nontrivial relationship; see Mathematical descriptions of opacity.

In biochemistry, the extinction coefficient of a protein at 280 nm depends almost exclusively on the number of aromatic residues, particularly tryptophan, and can be predicted from the sequence of amino acids.^[2] If the extinction coefficient is known, it can be used to determine the concentration of a protein in solution.

When there is more than one absorbing species in a solution, the overall absorbance is the sum of the absorbances for each individual species (X, Y etc.):

,

The composition of a mixture of N components can be found by measuring the absorbance at N wavelengths (the values of ε for each compound at these wavelengths must also be known). The wavelengths chosen are usually the wavelengths of maximum absorption (absorbance maxima) for the individual components. None of the wavelengths must be an isosbestic point for a pair of species. For N components with concentrations c_i and wavelengths λ_i, absorbances A(λ_i) are obtained:

.

This set of simultaneous equations can be solved to find concentrations of each absorbing species.

The molar extinction coefficient is directly related to the Absorption cross section σ via the Avogadro constant:

.

In units of cm^{2 [3]}

The molar absorptivity is also closely related to the mass attenuation coefficient, by the equation

(Mass attenuation coefficient)×(Molar mass) = (Molar absorptivity).

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