A novel-type dual-probe measuring device has been employed for investigating the effect of liquid viscosity on the dyanmics of bubble bed formation and on the values of bubble bed voidage and bubble frequency. Experiments were carried out in a bubble column reactor 0.152 m in diameter using aqueous solutions of saccharose within the viscosity range 1-110 mPa s. Two distributing plates with hole diameters of 0.5 and 1.6 mm and a free plate area ratio of 0.2% were alternatively used to identify the effect of primary gas dispersion in gas-liquid systems with viscous liquid phases. The experimental results demonstrate the existence of different dynamics of bubble bed formation at different levels of liquid viscosity and yield a critical viscosity mu(L) = 30 mPa s, indicating the appearance of a bimodal bubble population accompanied with an increase of gas holdup during the aeration due to an increasing contribution of small bubbles (d(b) < 1 mm) to the overall gas holdup. Increasing liquid viscosity influenced the stability of the homogeneous bubbling regime unfavourably and eventually (at mu(L) greater-than-or-equal-to 8 mPa s) eliminated the effect of primary gas dispersion on bubble bed hydrodynamics. The dependence of gas holdup on liquid viscosity in turbulent bubble beds exhibited a maximum at mu(L) congruentto mPa s followed by a sustained decrease of gas holdup with increasing viscosity up to mu(L) congruent-to 30 mPa s. Subsequent increase in bubble bed voidage with liquid viscosity can be ascribed to the increasing contribution of small bubbles to the overall gas holdup.