Hydrate formation kinetic modelling studies reported so far mainly concentrates on pure water-gas systems in stirred-tank batch environments. This work proposes a model for gas hydrate formation kinetics of a hydrate promoter-water-natural gas system in a semi-batch reactor assuming steady-state, isothermal and isobaric conditions. The hydrate formation kinetics was modelled after extending the recent method proposed by Kashchiev and Firoozabadi (J. Crystral Growth 241 (2002a) 220; J. Crystal Growth 243 (2002b) 476; J. Crystal Growth 250 (2003) 499) for a single component gas-water system to a multi-component gas-water-additive system. The extended Kashchiev and Firoozabadi model was applied for a semi-batch spray reactor here for the first time. The hydrate formation experiments were carried out in a pilot plant spray reactor at three different pressure-temperature regimes to determine the actual hydrate formation kinetics in the spray reactor. The experiment results were then used to finetune the adjustable parameters to facilitate accurate model predictions. (C) 2004 Elsevier Ltd. All rights reserved.