This work describes an experimental investigation about the coalescence of pairs of gas slugs rising in vertical columns of liquids covering a wide range of liquid viscosities. The experiments were performed in columns with 19, 32 and 52 mm of internal diameter and slug coalescence was followed by means of a new experimental technique, based on the signals of differential pressure transducers. Important data reported in this work are : -the minimum distance between slugs above which there is no interaction, l(min); -the velocity of approach of the trailing slug as a function of its distance to the leading one. Values of l(min) are shown to be related to the Row pattern in the wake of liquid behind gas slugs and correlations are provided for the determination of l(min); in all cases l(min) is about four times the wake length, l(w).