Decomposition in landfills is erratic and influenced primarily by moisture, inoculum, oxygen and accumulation of inhibitory fermentation products. These parameters may be optimized for decomposition by use of controlled landfill cells or bioreactors. This work evaluated the effect of combined-phase and two-phase anaerobic digestion of municipal solid waste (MSW) in laboratory leachbed (LB) cells. Phase separation resulted in increased rates of decomposition and greater process stability. Leachate recycle with an upflow sludge anaerobic reactor resulted in improvement of performance in a LB reactor which was attributed to removal of inhibitory fermentation products and buffering of acids in the leachate. The effects of aeration and leachate flooding of the LB were also studied. Intermittent aeration of the LB enhanced phase separation but inhibition of methanogenesis was not sustainable beyond six days. Conversion of organic matter was similar in the anaerobic and aerated LBs.