Beer's law and linear superposition of absorption spectra are known to be valid in dilute solutions. In this work it is shown with two examples that in a condensed phase conducting polymers are subject to the frequency-dependent local field of lightwave, also called dielectric effect, which influences the absorbance, the position of the absorption maximum, and the band width. The consequence is the apparent nonlinear behavior of absorption spectra and disappearance of isosbestic points in two-component systems. If the refractive index and absorbance as the function of wavelength are available, an appropriate correction can be made by using the Lorentz-Lorenz approximation for the local field. A nearly complete recovery of the isosbestic point is observed in the case of poly(3-methylthiophene) by using this local field approximation, casting some doubts on the conclusions previously drawn on the basis of these spectral data.