AbstractPseudo‐first order chemical absorption was used as the basis for determining the effective interfacial area available for mass transfer in gas‐liquid, cocurrent, upward pipe flow. The study, which was performed in a 2.54 cm I.D. tube, encompassed the slug and froth flow regimes.The interfacial area displayed a strong dependence upon the coordinates of the flow regime map proposed by Oshinowo and Charles(16). In addition, the systematic variation of surface area could be rationalized on the basis of flow regime geometry. Unfortunately the data could not be used to assess previous surface area models due to the unresolved diameter dependence of these models.Observations are also made concerning the apparently significant effect of chemical composition on interfacial surface area, and concerning the inadequacy of the popular methods used for estimating frictional pressure drop in vertical two‐phase