ABSTRACT A combined theoretical and experimental investigation to study the behaviour and characteristics of gas-liquid two-phase flow in horizontal pipelines is described in this paper. A simple theoretical model was developed to provide predictions of liquid hold-up in equilibrium stratified two-phase flow regimes and transition boundaries into slug flow. In order to calculate the liquid hold-up, the pressure drop equations were considered. The pressure drops in the two phases together with the shear stresses at the walls and interface were solved numerically. The criterion employed for predicting the onset of slug formation is wave instabiltiy which occurs when the wave velocity is imaginary. The experimental study was conducted over a wide range of flow conditions to investigate pressure loss, liquid hold-up, flow pattern transitions and slug/bubble characteristics. The results presented in the paper are only flow pattern transitions and slug/bubble characteristics. The experiments were conducted on 0.2 and 0.4m, diameter, 400m long loops at BHRA for a range of superficial air and water velocitie's up to VSG = 24m/s and VSL = 2n/s. The agreement between the experimental observations and the predictions of the transition into slug flow is considered excellent. Based on the average experimental data generated from these loops and from a separate study conducted on different fluids and pipelines, various empirical correlations for calculating the slug/bubble characteristics have been developed. Comparisons were also made between the measurements and current design methods, thus stating their range/limitations of applicability.