AbstractGas density affects the gas holdup in bubble columns. Even at atmospheric pressure, different gases produce varying gas holdups. This contribution explores the causes for variations in gas holdups occurring for different gases. Differences in the bubble size distribution resulting from varying rates of bubble breakup and coalescence as well as dissimilar bubble rise velocities are the most likely causes of differences. Correlations that include a term for gas density are able to qualitatively predict the effect of changing gas density, but no correlation can consistently predict the gas holdup for multiple systems.