We report on measurements of X-ray excited charge Transfer luminescence occurring in Yb3+-doped yttrium aluminium garnet (YAG:Yb) and ytterbium aluminium garnet (YbAG). The temperature dependence of luminescence was investigated in variously Yb3+-doped crystals. The fluorescence intensity and decay time drop drastically at low temperatures (T < 100 K), where thermoluminescence peaks are observed. The strong afterglow occurring in the temperature range of the glow peaks reflects trapping effects. Data show that YAG:Yb crystals are potentially good scintillators for solar neutrino detection, and they can still be optimised in terms of efficiency and lifetime.