Mixtures are azeotropic if they can be distilled (or condensed) without a change of composition(1). The existence of azeotropes in multicomponent mixtures in the absence of chemical reactions is well understood phenomenologically(2,3) and theoretically(4,5). Azeotropes place a fundamental limit on the compositions attainable in mixtures by fractional distillation, but they can in some cases be ’broken’ by carrying out chemical reaction and separation simultaneously rather than sequentially(6-9). Here we report the discovery of a boiling state of constant composition and temperature in a mixture of acetic acid, isopropanol, isopropyl acetate and water that is simultaneously in both reaction and phase equilibrium, These states, which we call reactive azeotropes, were predicted recently(10,11). Without reaction, the mixture exhibits three two-component azeotropes, one three-component azeotrope but no four-component azeotrope; the last appears only under equilibrium reaction conditions. These findings may constrain technologies in which reaction and separation are conducted simultaneously, for example by limiting the conditions under which an azeotrope can be broken by chemical reactions to yield a high-purity product. In other cases the presence of a reactive azeotrope may be advantageous(9).