Protonic conduction in Ba2In2O5 was studied by conductivity and EMF measurements as a function of temperature and oxygen activity. Evidence for protonic conduction was found in this material in moist atmospheres, especially at low temperatures. Three regions of protonic conduction in the Arrhenius plot have been observed and analyzed. At low temperatures (T < 400 degrees C) the protonic conductivity is history dependent, and the proton mobility enthalpy (Delta H-H,H-m= 0.77 eV) has been obtained from conductivity measurements. At intermediate temperatures between the oxygen order-disorder transition temperature (T-d = 925 degrees C) and 400 degrees C, equilibrium with moist atmospheres can be reached. Considering the oxygen vacancy and proton formation mechanisms, the proton formation enthalpy for consuming one H2O and creating two protons Delta H-H,H-F = -0.90 eV) has been obtained. The observed proton conductivity transition at the oxygen order-disorder transition temperature directly confirms the proton formation mechanism by incorporation of H2O molecules into oxygen vacancies.