Experimental and numerical studies concerning the formation of droplets of a viscous fluid in a less viscous immiscible one (viscosity ratio of 533), containing surfactant, were conducted in a flow-focusing square microchannel. Numerical and experimental data from three flow regimes were analysed: threading, dripping and jetting. The results were analysed qualitatively and quantitatively and also compared to correlations from the literature. The droplet sizes and the thread lengths obtained experimentally deviate from the ones obtained for surfactant free systems reported in the literature. The roles of the surfactant mass transport limitations and of the interface rheology are analysed and discussed. The numerical code, based on a coupled level-set and volume of fluid method, successfully predicts droplets sizes in surfactant free systems in almost all the conditions studied. The limits of the numerical code to accurately predict the size of the droplets, particularly in the monodispersed dripping regime, were established. If surfactant is present, the numerical code fails to predict the droplet size under certain flow conditions. (C) 2018 Elsevier Ltd. All rights reserved.