Catalytic hydrogenations reactions are frequently conducted in dead-end multiphase stirred reactors with the reactant dissolved in either an alcohol, water, or a mixture of the two. In such systems, the rate of gas-liquid mass transfer which depends on bubble size may well be the overall rate-limiting step. However, a study of bubble sizes across the whole range of solvent compositions from entirely water to entirely organic has not been reported. In light of this, a systematic study has been made in a 3 L, dead-end vessel containing 1 vol % of air dispersed by a Rushton turbine in water, isopropanol (IPA) and mixtures of the two, with and without 2-butyne-1, 4-diol simulating a reactant. In the single component solvents, though the interfacial tensions are very different, irregular, relatively large bubbles of similar sizes were observed, with a wide size distribution. In the mixed aqueous/organic solvents, and especially at the lower concentrations of IPA, the bubbles were spherical, much smaller with a narrow size distribution. The addition of the reactant to the mixed solvents had little effect on the mean size, shape, or distribution. However, addition to water led to small spherical bubbles of narrow size distribution. This is an abstract of a paper presented at the AIChE Annual Meeting and Fall Showcase (Cincinnati, OH 10/30/2005-11/4/2005).