The Bouguer—Lambert—Beer absorption law is one of the key laws in spectrophotometric research. Over the last few decades, the researchers have pointed out a number of factors that influence the validity of this law. The paper shows that for the weak electrolytes with the formal linear dependence of the absorbance on the concentration А = f(C) with the correlation coefficient r ≈ 0.99 and more, a major deviation of the experimentally determined molar absorption coefficient εobs from the Bouguer—Lambert—Beer Absorption Law can be observed, and it depends on the concentration of the electrolyte solution.The article presents experimental and calculated data illustrating the reasons that lead to the instability of the molar absorption coefficient εobs ≠ const in the solutions with the concentration of less than 10—3 mol/l. It is suggested that, for precise spectrophotometric measurements, the εobs = f(C) dependance, rather than А = f(C), is the most informative one. First time a theoretical model is offered, that reliably describes the εobs = f(C) dependences for the electrolytes of various strengths, which can be instrumental for the further, more detailed studies and analyses of equilibrium in electrolyte solutions, using the innovative method of concentration spectrophotometry.