High viscosity oil/air slug flow is investigated both experimentally and theoretically. The behaviour with three different oils, whose viscosities range from 0.037 Pa s to 0.804 Pa s, is investigated in a horizontal pipe with 0.022 m I.D. and 9 m length. A statistical approach is adopted during the tests so to collect a meaningful amount of data and to build the probability density functions for slug lengths. A new correlation to model their dependence on superficial gas velocity is introduced. The length correlation along with a corrected version of the Nicklin (1962) correlation is used as an input into a developed mechanistic model, which is able to predict the pressure drops along the slug unit. The Modified Two Fluid model (Ullmann and Brawler, 2006) is used as closure relation for shear stresses in the stratified region. The model is validated against the pressure drop data obtained from performed experiments showing good accordance with them and providing detailed pieces of information about the mechanics of the flow regime. (C) 2016 Elsevier Ltd. All rights reserved.