Effects of electrolytes on bubble coalescence are investigated experimentally. Collisions of bubbles with free surface of three kinds of liquids, namely, super purified water, NaCl solution and KCl solution, are observed by using a high speed camera. It is found that the numbers of bounces on a free surface in electrolyte solution are larger than those in super purified water with the same equivalent bubble radius. This result qualitatively indicates that electrolyte works to prevent bubbles from coalescing. It is also revealed that a bubble can coalesce if the Weber number, which is based on radius of curvature of fore side of the bubble and approach velocity just before collision with free surface, is less than a critical value. The decrease of the critical Weber number, which corresponds to the inhibition of the coalescence, is observed with the increase in the concentration of the electrolyte solutions.