The average diameter, d, of bubbles generated from a porous plate in pure liquid and in aqueous solutions of inorganic electrolyte were measured at various gas flow rates by taking photographs of bubbles the size of which was controlled by bubble coalescence on and above the plate. For the extreme case when coalescence occurred at the maximum rate such as in water and in pure organic liquid, the empirical equations for d based on previous data and those from this experiment are proposed. The addition of inorganic electrolyte to water reduced the coalescence and thus simultaneously reduced d. The average bubble diameter in the aqueous solutions is well correlated to the experimental conditions with an empirical equation including the non-dimensional group proposed by Marrucci and also corresponding to the controlling factor of the bubble coalescence.