Aims: Decomposition of solid waste is microbially mediated, yet little is known about the associated structure and temporal changes in prokaryotic communities. Bioreactors were used to simulate landfill conditions and archaeal and bacterial community development in leachate was examined over 8 months. Methods and Results: Municipal solid waste (MSW) was deposited in laboratory bioreactors with or without biosolids and combustion residues (ash). The near-neutral pH fell about half a log by day 25, but recovered to similar to 7 center dot 0 by day 50. Cell concentrations in bioreactors containing only MSW were significantly higher than those from co-disposal bioreactors. Archaeal and bacterial community structure was analysed by denaturing gradient gel electrophoresis targeting 16S rRNA genes, showing temporal population shifts for both domains. mcrA sequences retrieved from a co-disposal bioreactor were predominantly affiliated with the orders Methanosarcinales and Methanomicrobiales. Conclusion: Regardless of waste composition, microbial communities in bioreactor leachates exhibited high diversity and distinct temporal trends. The solid waste filled bioreactors allowed simulation of solid waste decomposition in landfills while also reducing the variables. Significance and Impact of the Study: This study advances the basic understanding of changes in microbial community structure during solid waste decomposition, which may ultimately improve the efficiency of solid waste management.