Twelve hydrothermal pre-treatment combinations of temperature (150 and 180°C), time (10 and 20min) and acid catalyst (no catalyst; H2SO4 at 2% w/w immediately before steam cooking or in 24-h pre-soaking) were tested to assess their effects on methane yield of Giant reed biomass vs. untreated control. A batch anaerobic digestion was conducted with 4gVSl−1 at 53°C for 39days. Untreated biomass exhibited a potential CH4 yield of 273mlg−1 VS; the four pre-treatments without acid catalyst achieved a 10%, 7%, 23% and 4% yield gain in the respective temperature/time combinations 150°C/10min, 150°C/20min, 180°C/10min and 180°C/20min. Conversely, the eight pre-treatments with H2SO4 catalyst incurred a methanogenic inhibition in association with high SO42- concentration in the hydrolysate, known to enhance sulphate reducing bacteria. Furfurals were also detected in the hydrolysate of five strong pre-treatments with H2SO4 catalyst.