Ba4Nb2O9 and Ba4Ta2O9 decompose upon heating in a CO2-rich atmosphere. Here, we report further in situ synchrotron X-ray diffraction studies under CO2-free atmospheres of variable humidity and under vacuum and show that this strongly influences the reconstructive phase transitions normally seen. Under normal atmospheric conditions, alpha-Ba4Nb2O9 transforms to gamma-Ba4Nb2O9 at 1493 K and alpha-Ba4Ta2O9 transforms to 6H-Ba4Ta2O9 at 1513 K. Under wet argon, these transitions shift to lower temperatures of 1453 K and 1443 K for Ba4Nb2O9 and Ba4Ta2O9 respectively. Upon heating both the alpha samples under vacuum, no phase transition is observed at all up to 1723 K. Heating 6H-Ba4Ta2O9 under vacuum saw the monoclinic -> hexagonal symmetry-transition temperature increase from 1050 K under air to 1700K. The significance of these observations in terms of possible practical applications of their mixed oxide ionic, protonic and electronic conduction properties are discussed. (C) 2012 Elsevier B.V. All rights reserved.