The recent unsolved wake theory to describe bubble coalescence by Colella et al. [1999. A study on coalescence and breakage mechanisms in three different bubble columns. Chem. Eng. Sci. 54, 4767-4777] is solved analytically to give exact values of coalescence time in terms of bubble size and relative rise velocity. The solution is expressed using the tabulated exponential integral function. (C) 2009 Elsevier Ltd. All rights reserved.