Experimental data are presented on the axial development of gas-liquid flows in a 42-m long, 0.048-m ID tube system. Different transitional models for slug, churn and annular flow regimes were compared with visual observations and showed reasonable agreement for locations L/D = 560 and 820. The behavior of liquid holdup and frequency of flow structures (large waves, disturbance waves) were also analyzed for three different axial positions (L/D = 102, 521 and 815), over a range of pressure between 1.4 and 5.2 bar, liquid mass fluxes ranging from 17 to 319 kg m(2)-s and dimensionless gas velocities from 0.05 to 1.6. The liquid holdup showed significant axial variation at all monitoring locations for higher liquid mass fluxes, while it appeared that a kind of developed flow was reached at L/D = 521 for lower liquid mass fluxes. The flow structure frequency did not exhibit significant axial variation for dimensionless gas velocities (Wallis parameter) between 0.2 and 1.6. (c) 2013 Elsevier Ltd. All rights reserved.