An experimental study is presented about the interaction between two slugs rising through a co-current flowing liquid in vertical tubes with 22, 32 and 52 mm internal diameters. For the conditions studied the liquid flow pattern in the wake of the slugs was turbulent and the main liquid flow regime was either laminar or turbulent. When the liquid flow regime is turbulent the minimum distance between slugs above which there is no interaction is about 5D. For lower distances the slugs coalesce during their rise. When the liquid flow regime is laminar two different situations are observed : -if the ratio between the average velocity in the full developed film around the slug and the average velocity in the main liquid is greater than 25, coalescence between slugs is observed. The minimum distance between slugs above which there is no interaction is about 10D; -if the value of that ratio is lower than 25 and the initial distance between slugs is greater than the wake length of the leading slug, the distance between slugs increases during their rising in the column and slugs coalescence is not observed. This behaviour has never been referred in the literature. The wake length is found to be 5D whatever is the main liquid flow regime.