The mechanism of biogas production from camel, cattle, goat, horse and sheep dung slurries has been investigated by adopting nine mechanistic models which are used in the kinetics of thermal decomposition of solids. To achieve this, the volume of biogas produced was recorded at 24-h intervals over a period of 13 weeks and converted into the fraction of gas produced (alpha). For each model, g(alpha), a straight line plot was attempted using the equation: g(alpha) = kt(0)-kt, and linear regression analysis was used to establish the 'best-fit' model, which was then considered as descriptive of the rate-limiting step in biogas production. The analysis revealed that g(alpha) = alpha is the 'best-fit' model for all the slurries. This implies that nucleation and subsequent growth of the nuclei is the rate-determining step. This, in the case of the experimental design used in this study, is synonymous to the formation and subsequent growth of biogas bubbles in the digester. Thus, the rate of biogas production from animal dung slurries could be said to be dependent on the nucleation centres in the slurry. (C) 2003 Elsevier Ltd. All rights reserved.