The aim of the study was to evaluate the possibility of using a high rate anaerobic process to convert the soluble hydrolysis and acidification products from a first phase solid substrate anaerobic digester treating the organic fraction of municipal solid waste (OFMSW). To achieve this a comparative evaluation of the kinetics methods for predicting the effluent (S_e) soluble substrate concentration was undertaken. The methodology was developed by conducting anaerobic treatability studies on a readily degradable soluble wastewater, this was then extended to studies using a solids free leachate derived from the hydrolysis and acidification of OFMSW, and finally to a mixed soluble/suspended solids phase leachate produced by a high rate hydraulic flush bioreactor treating OFMSW. Using the readily degradable wastewater two sets of experiments were undertaken to assess the kinetics of both a batch operation and fed-batch operation. These treatability studies were carried out at a laboratory scale in stirred tank reactors. Batch operation using this wastewater was inherently unstable and it was demonstrated that the problems were due to both a nitrogen deficiency in the wastewater and a lack of natural buffering capacity. Stable operation could be maintained by supplementing the buffering capacity by daily addition of NaHCO₃ and NH₃HCO₃ at 250-400 mg.I⁻¹.d⁻¹ and 10 mg.I⁻¹.d⁻¹ respectively to the batch system, this also provided sufficient nitrogen to maintain a healthy bacterial population. The kinetics of the batch reaction were best described using the equation S_e = S_o^kt (S_o = influent; t=time), the constant k was equal to 0.02h⁻¹ under normal opening conditions. For batch operation, an estimate of the maximum gas production (G_m) could be make using the specific function G=G_m^k/t (G= gas production). Statistically, this gave a better estimate of G_m than other known methods, in addition the method developed was more straightforward. For the fed-batch reactor treating the readily degradable wastewater, a Michaelis-Menten kinetic approach was adopted and the reaction was proven to be of a first order, except at high loadings (>1.3 kg.m³.d⁻¹). The effluent soluble substrate could be predicted with confidence using the equation S_e= S_o / [1 + kHRT] (HRT - hydraulic retention time). An organic loading rate (OLR) of 1.4 kg.m³.d⁻¹ could be treated at a removal efficiency (E_r) of 92%. For a fed-batch operation, the constant k in the first order model when applied to the readily biodegradable wastewater was equal to 1.25d⁻¹. The coefficients r_m and K_m for the Michaelis-Menten kinetic equation were equal to 1700 mg.l⁻¹.d⁻¹ and 310 mg.l⁻¹.