Gas holdups and volumetric mass transfer coefficients were studied in a bubble column varying the gas density by a factor of up to 300. The gas density effect increased with the gas velocity. A maximum effect was reached where the gas holdup (in water and in 0.8 M sodium sulfate solution) varied with the gas density to the power of 0.28. The corresponding effect on the volumetric mass transfer coefficient k(L)a’ was described with a power of 0.46. The same strong influence on k(L)a’ holds also for highly viscous pseudoplastic xanthan gum solutions and concentrated slurries of kieselguhr (less than or equal to 12.5 vol%) and alumina (less than or equal to 18 vol%).