The results and discussion of a study of the effects of tube diameter on vertical slug flow, specifically as it relates to 3 -25 mm airlift pump performance are presented. The theory previously presented by Nicklin (1963), is extended into this range of tube diameters by taking into account the effects of surface tension on bubble rise velocity. Differences are noted between the rise velocity of a single gas slug and a train of gas slugs in small vertical tubes. Comparisons are made between experimental observations and theoretical predictions.