The solid solubility of Ho3+ in BaTiO3 red under the reducing conditions of a nitrogen atmosphere containing 10% of hydrogen was studied by quantitative electron-probe microanalysis using wavelength-dispersive spectroscopy. The solubility was found to depend mainly on the starting composition. In the TiO2-rich samples, the solubility of the Ho3+ donors at the Ba lattice sites was measured to be similar to 4 mol% Ho-Ba (Ba0.96Ho0.04TiO3-delta at 1400 degrees C), which is much higher than after sintering in air. In the BaO-rich compositions, the solubility of the Ho3+ acceptors at the Ti sites (solubility limit similar to BaTi0.91Ho0.09O2.955-delta(V-O(center dot center dot))O0.045+delta at 1500 degrees C) decreased compared with that in air. Like with sintering in air, under reducing conditions, the highest solubility of holmium in BaTiO3 was determined when it was incorporated at both lattice sites (solubility limit similar to Ba0.91Ho0.09Ti0.85Ho0.15O2.97-delta(V-O(center dot center dot))(0.03+delta)).