Two-dimensional correlation spectroscopy (2D-COS) is a highly sensitive technique for detecting deviations from the Beer–Lambert approximation through asynchronous spectra. In this study, we apply 2D-COS to examine such deviations in the context of this approximation. While conventional molecular IR spectroscopy literature suggests a linear correlation between absorbance, molar concentration, and sample thickness, a more rigorous analysis, supported by electromagnetic theory, demonstrates that this assumption does not hold, even under ideal conditions. As a result, disproportionate spectral changes, caused by interference effects, give rise to distinct patterns in asynchronous 2D-COS IR spectra. To illustrate this, we investigate the thickness dependence of absorbance in poly(methyl methacrylate) (PMMA) layers deposited on calcium fluoride (CaF2) and Si. Our findings reveal systematic variations not only in absorbance values but also in band shapes and peak positions. 2D-COS emerges as a powerful tool for identifying and analyzing these patterns.