A structured mathematical model of competition between sulphate reduction and methanogenesis in anaerobic reactors has been developed. This model includes multiple-reaction stoichiometry, microbial growth kinetics, conventional material balances for an ideally mixed reactor; liquid gas interactions and liquid phase equilibrium chemistry. The model agrees well with existing experimental studies of this competition in anaerobic reactors. Hypothetical computer simulations are presented to illustrate the influence of hydraulic retention rime, SO42-: COD ratio, initial proportion of sulphate reducing and methanogenic bacteria in seed sludge, sludge retention, hydrogen sulphide inhibition and other factors on the outcome of this competition,