Vertical electrolysers with parallel-plate electrodes and with a narrow interelectrode gap or with narrow gaps between the membrane and each of the electrodes are used industrially to produce gases. Conductivity measurements were carried out with a small conductivity cell built within one compartment of the cell and just above the top of the working electrode. KOH solutions were used as electrolytes and gas bubbles were evolved at a 20-segment electrode. The effect of various parameters, including current passed through the 20-segments electrode, flow rate of liquid, temperature and nature of gas evolved, was studied. From the normalized conductivity measured, the gas voidage epsilon was obtained. It was found that for a bubbly flow the gas voidage just above the gas-evolving electrode is given by the relation : epsilon/(epsilon(max) - epsilon) = 1.4 Q(g)/Q(1) where epsilon(max) is the maximum voidage for gas bubbles in a solution; epsilon(max) = 0.69 for oxygen as well as hydrogen in a concentrated and a dilute KOH solution at different liquid flow rates. Q(g) is the volumetric flow rate of gas saturated with water vapour and Q(1) is the volumetric flow rate of liquid, This relation can be used to calculate the voidage distribution in the bulk solution of the cell compartment with a gas-evolving electrode.