莫耳消光系数，又称莫耳吸光度，是衡量化学物种吸收特定波长光强度的度量单位。此为物质的固有性质。根据比尔-朗伯定律，A = εcl，试样的实际吸光度A，与路径长l、物质浓度c有关。ε的单位通常是Mcm或L molcm。因为1公升合1000cm，有时1000倍大的数值在旧表示法中常使用cmmol。

在生物化学里，蛋白质在280nm的消光系数几乎全由芳香族残基，由其是色氨酸所决定，可由胺基酸串行预测。 若消光系数已知，可以决定溶液中蛋白质浓度。

另一个消光系数的度量单位是E1%，称为质量消光系数。E1%是1%质量百分浓度溶液的吸光度，单位为gL cm。ε和E1%的换算可使用下列公式：ε = (E1%*分子量)

The molar attenuation coefficient is a measurement of how strongly a chemical species attenuates light at a given wavelength. It is an intrinsic property of the species. The SI unit of molar attenuation coefficient is the square metre per mole (m/mol), but in practice, it is usually taken as the M⋅cm or the L⋅mol⋅cm. In older literature, the cm/mol is sometimes used with corresponding values 1,000 times larger. In practice these units are the same, with the difference being expression of volume in either cm or in L. The molar attenuation coefficient is also known as the molar extinction coefficient and molar absorptivity, but the use of these alternative terms has been discouraged by the IUPAC.