The effect of addition of electrolytes on gas hold-up of air/water system was investigated experimentally in a laboratory scale bubble column. The experiments were carried out with four electrolytes, namely, NaCl, MgSO(4)center dot 7H(2)O, Na(2)SO(4) and CaCl(2)center dot 2H(2)O and the concentrations of the solutions were varied from 0 to 0.3 mol/l. Enhancement of gas hold-up was observed for all four electrolytes at concentrations less than 0.1 moL/l. With the increase in concentration, the gas hold-up showed two different trends; in Na(2)SO(4) and CaCl(2)center dot 2H(2)O solutions, gas hold-up formed a sharp peak after the enhancement and leveled off at a value somewhat higher than that in water, whereas in NaCl and MgSO(4)center dot 7H(2)O solutions, gas hold-up leveled off immediately after the enhancement without forming any peak. Experiments were also conducted to measure the surface tensions of the solutions with special focus in the low concentration region. A strong relation between the gas hold-up enhancement and the change of surface tension with the addition of electrolyte was found. It was also observed that the concentration at which maximum value of C(d sigma/dC)(2) i.e. (concentration x surface tension gradient with respect to concentration(2)) is obtained corresponds to the concentration at which maximum gas hold-up enhancement occurs. (C) 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.