This paper studies the liquid holdup within the mixing zone of the slug. The results of the study show that the liquid holdup begins at the liquid holdup of the liquid film before the slug and then increases until the end of the mixing zone is reached. Once past the mixing zone of the slug, the average liquid holdup becomes constant. As the height of the liquid film and/or viscosity increases, so does the liquid holdup at any given film Froude number. Since the liquid holdup becomes constant once past the mixing zone of the slug, the mixing zone length was determined for the film Froude numbers studied. The results show that the mixing zone length increases linearly with film Froude number and is independent of the viscosity of the liquid in the slug for a viscosity range of I to 16.6 cP.