Herein we describe the synthesis and luminescence of nanosized (Lu1-y-xGdx)2O2S:Tby and (Lu1-y-xGdx)2O3:Tby phosphors with y = 0.1 mol% Tb3+ and y = 2 mol% Tb3+ and x ranging between 0 and 1. The concentration of Gd3+ (x) was varied in steps of 0.1 (molar ratio Gd3+). The samples at 0.1 < x < 0.7 contained a mixture of (Lu1-xGdx)2O3:Tb3+ and (Lu1-xGdx)2O2S:Tb3+, while the samples at x = 0 contained only Lu2O3:Tb3+. At 0.1 < x < 0.7 Lu2O2S:Tb3+ and Gd2O2S:Tb3+ did not form a solid solution, but rather crystallised into two slightly different hexagonal structures. This behaviour has been explained in terms of segregation of Lu and Gd between the oxide and oxysulfide phases: the oxide phase is more Lu-rich whereas the second oxysulfide phase is more Gd-rich. The photoluminescence spectra of the phosphors with 0.1 mol% Tb3+ showed a modest colour change of the fluorescence light from cyan to green when x was increased from 0 to 1, whereas the samples of the series with 2 mol% Tb3+ yielded essentially green light. From this analysis it was concluded that the colour change of (Lu1-xGdx)2O2S:0.1%Tb3+ is caused by increasing energy transfer of the 5D3-level of Tb3+ to the charge transfer band of (Lu1-xGdx)2O2S:Tb3+ upon increasing x. Since the samples with 100% Lu consisted of pure cubic Lu2O3:Tb3+, we had the opportunity to also study the symmetry-related PL of this compound. From this study we concluded that the C2-C3i doublet of the Tb3+ 5D4 → 7F5 transition behaves in the same way as the corresponding doublet in cubic Y2O3:Tb3+.