Isotopologues are molecular species differing only in their stable isotope composition. For example, the isotopologues of water include HOH, DOD, and HOD. The possibility of reactive azeotropy among isotopologues of the same molecule is explored within the ideal gas/ideal solution assumption. The chemical equilibrium enabling azeotropy may involve isotope exchange reactions or may involve more complicated dissociation/solvation chemistry. Azeotropy among isotopologues appears possible in the presence of isotope exchange reactions and with simple dissociation or combination reactions. However, for such chemical reactions, the isotope fractionation factors must be unity, that is, isotope ratios are identical in the vapor and liquid phases. This is not consistent with experimental observations that isotope fractionation factors can differ from unity at an azeotrope. More complex solvation chemistry, wherein the isotopic composition in the solvation shell differs slightly from that of the bulk solvent, can lead to an azeotrope for which the isotope fractionation factor is not unity. Possible azeotropy among the isotopologues of water is then considered. Experiments to directly quantify water isotopologue compositions at the elevated temperatures and pressures anticipated for azeotropy are described. The paper concludes with a discussion of the potential impact of azotropic isotopologues on the azeotropic extraction and isotopic analysis of soil water and on the interpretation of isotopic signatures in geological and cosmological samples. (C) 2019 Elsevier B.V. All rights reserved.