A comprehensive treatment of all sources of pressure drop within intermittent gas–liquid flows is presented. Pressure loss associated with the viscous dissipation within a slug is calculated, and the presence of dispersed bubbles in a slug is accounted for, without recourse to the widely used assumption of homogenous flow. Experiments were conducted to measure pressure gradient within two air–water pipes of 32 and 50mm internal diameter at 0 and +10° inclination to the horizontal. The results show that existing intermittent flow models predict pressure gradients considerably lower than were observed. The present model predicted pressure gradients in good agreement with all the measurements and this was achieved without introducing any additional reliance on empirical information.