Void fraction, Taylor bubble and liquid slug lengths, and slug frequency are parameters essential to any description of the structure of slug flow. In the present study, these parameters were extracted from the time series of cross-sectionally averaged void fraction obtained from two vertical facilities having similar internal pipe diameter but significantly different axial lengths; 51.2 mm/3.5 m and 52.3 mm/9 m. In order to study slug flow, the flow rates for which it occurred were first identified. To investigate the effect of flow development on slug characteristics measurements were carried out at several axial locations from the mixing section for both facilities. For slug frequency, a new correlation including the effect of the axial length has been proposed and assessed using previously published data. (C) 2009 Elsevier Ltd. All rights reserved.