Our purpose is to study the effect of degradation processes on the optical response of ancient paper specimens. The paper samples under study originate from several European countries, date back to the XV and XVI centuries and show different states of degradation. Paper optical response is checked by reflectance spectroscopy in the wavelength range of 250–1100 nm. Similar spectral forms are shown by paper specimens displaying a broad range of widespread discoloration, of different manufacture, geographical origins and ages. A similar form is also observed by measuring foxing spots of several intensities on a single foxed sheet. Moreover, the entire spectra set shows a clear connection between the paper’s state of degradation and a monotonic transformation of the spectral curves, suggesting the existence of uniform degradation effects implied in the optical response of both foxed as well widespread discoloured papers. These results suggest for usage of optical reflectance as a general and non-destructive method for degradation calibration.