Herein we describe the synthesis and spectral analysis of nanosized (Y1-xGdx)2O2S:Tb3+ phosphors between x = 0 and x = 1 with 0.1 and 2 mol% Tb3+. The concentration of Gd3+ was varied in steps of 0.1 (mole ratio Gd3+). X-ray diffraction analyses confirmed the purity and composition of the phosphor materials. The photoluminescence spectra of the series of phosphors with 0.1 mol% Tb3+ showed a colour change of the fluorescence light from blue to green when x was increased from 0 to 1, whereas the samples of the series with 2 mol% Tb3+ yielded essentially green light. These phenomena could be explained in terms of energy transfer from the Tb3+ 5D3-level at 26 316 cm-1 to the CT-minimum of (Y1-xGdx)2O2S:Tb3+ for the series with 0.1 mol% Tb3+, while for the series with 2 mol% Tb3+ cross relaxation between the 5D3, 7FJ and 5D4 levels of Tb3+ caused additional depopulation of the 5D3-level at low Gd3+ concentrations. Cathodoluminescence spectra recorded at temperatures between -170 °C and 20 °C confirmed the proposed energy flow and enabled the evaluation of the energy barrier for this energy flow. By modelling the energy flow we were able to show that the concentration of Gd3+ is critical in controlling the change in colour of the (Y1-xGdx)2O2S:Tb3+ for the series with 0.1 mol% and that the presence of just over x = 0.1 of Gd3+ is enough to switch to the energy transfer route present in the x = 1.0 parent phosphor. In addition the charge transfer band of (Y1-xGdx)2O2S:Tb3+ showed a red shift of about 6 nm upon increasing x from 0 to 1. This red shift has been described in terms of an electrostatic model that enabled a calculation of the rearrangement of the electrons during the charge transfer.