We study the evolution with doping of the optical spectra of the Cu–O planes in Ln2−xCexCuO4−δ (Lnlanthanide, mostly Nd) with 0≤x≤0.21. We analyse in detail the spectral features that appear between 0.7 and 5.2 eV in room-temperature ellipsometric measurements. The intensity and energy position of the observed spectral features show systematic dependences with the composition of the samples. Already light doping causes relevant changes in the line shape of the charge-transfer excitation, located at 1.55 eV, due to the interaction of localized and mobile carriers, confirming the excitonic character of this transition. The additional shoulder that appears at 2 eV has band character. The doping-induced feature near 4.3 eV is assigned to an intraionic Cu transition. Its smooth shift with x gives the progressive Fermi level rise with doping. The systematics of all the results supports the picture of a doped insulator.