Flow patterns, void fraction and friction pressure drop measurements were made for an adiabatic, vertical up-and-down, two-phase flow of air-water mixtures across a horizontal in-line, 5 x 20 tube bundle with a pitch-to-diameter ratio of 1.28. The flow patterns in the cross-flow zones were obtained and flow pattern maps were constructed. The data of average void fraction were less than the values predicted by a homogenous flow model and showed a strong mass velocity effect, but were well-correlated in terms of the Martinelli parameter X-tt and liquid-only Froude number Fr-LO. The two-phase friction multiplier data could be well-correlated with the Martinelli parameter.