Ta doping in Li4Ti5O12 (Li4Ti4.95Ta0.05O12) as function of different heat-treat atmospheres (oxidizing/reducing) was investigated and compared to Li4Ti5O12 to determine its effect on ionic/electronic conductivity and the charge compensating defects. Under oxidizing conditions Li4Ti4.95Ta0.05O12 was primarily an ionic conductor where the extra charge of Ta was compensated by Ti vacancies. Under reducing conditions Li4Ti4.95Ta0.05O12 was primarily an electronic conductor where the extra charge of Ta was compensated by an electron. The charge compensating defects were confirmed by sintering data.