The bubble diameter in two-phase flows is an important factor in determining the momentum, mass and heat transfer between the phases. In this paper we consider two main methods of prediction and, in the experimental measurements, take particular care to prevent surface contamination. We report average bubble diameters in bubbly flow for different gas-liquid systems in ducts of 7-, 10.9- and 13.2-mm diameter, smaller ducts, and larger energy dissipation rates, than generally used. The systems were mainly air-water and air-1-hexanol with limited measurements for air-perfluorohexane. The two theories are a simple equation previously proposed by Winterton and Obry, and Hinze's theory, which requires a method of calculating the two-phase energy dissipation. The method is explained and then applied to our own data and also to an extensive compilation of water data in the literature.