The biodegradation of waste placed in layers was studied in 500-ml lysimeters, without leachate recycle. All four became methanogenic within 100 days but the steady-state biogas output only reached 0.09 l kg(-1) d(-1), whereas an earlier study on a mixed feedstock had yielded 1.8 l kg(-1) d(-1). A novel physical model of solid-state digestion is deduced, implying a crucial role for mass transfer processes, with successful operation only within a limited range of diffusion rates.