There are numerous studies on two-phase anaerobic digestion of municipal solid waste (MSW) in the literature. Yet, in the majority of these studies, applied organic loading rates (OLR) were relatively low. This study differs from those in the literature in terms of high OLR application and focuses on the optimization of the acidification phase. Effect of high OLRs and pH values were investigated and optimum operational conditions for acidification were determined. Although conversion of particulate organics to soluble chemical oxygen demand ( COD) was achieved in the reactors, total COD values remained the same. Various types of organic acids were produced in the reactors and the selective production of organic acids in the anaerobic acidogenesis process was possible by controlling the organic loading rate and pH. Optimum OLR and pH values were determined as 15 g volatile solid (VS)/L . day and 5.5 +/- 0.1, respectively, for the MSW composition investigated.