Landfill continues to be the major method of Municipal solid waste (MSW) disposal in the UK and many other countries despite considerable efforts to limit its use. The EU Landfill Directive requires, amongst other things, that waste is treated to reduce its biodegradability prior to disposal to landfill. This pre-treatment is often achieved through what is generically termed mechanical-biological treatment. Predicting the biodegradability or degradation potential of these pre-treated wastes is important for the long term management and aftercare of landfill sites. To address this, a series of biochemical methane potential (BMP) tests have been undertaken to characterize the anaerobic biodegradation potential of two mechanically biologically treated (MBT) waste samples in terms of biogas yield, solids composition (loss on ignition, total carbon, cellulose, hemicellulose and lignin contents), and assessment of leachate characteristics during the biodegradation process. Experimental results from a long term study of MBT wastes treated to different standards are analyzed and compared. The relationship between biogas potential and solids composition was investigated, and carbon and nitrogen mass balances are discussed. The biogas potential was shown to correlate well with the ratio of cellulose plus hemicellulose to lignin, loss on ignition and total carbon content of the waste indicating a clear link between these parameters. The results indicate that solids composition of MBT wastes may provide a useful indication of the biodegradation potential. The mass balance indicates that a large proportion of carbon and nitrogen remain locked up in the waste material and is not released.