Large molecular refractory organic compounds (i.e., humic substances) were the major chemical oxygen demand (COD) components of aged raw landfill leachate. To investigate the behaviours of the large molecular refractory organic compounds when they were subjected to oxidation with ozone only (O 3 only) and ozone combined with hydrogen peroxide (O 3 /H 2 O 2 ), the aged raw landfill leachate first was filtered with 0.8 and 0.45μm pore size filters in series, then was sequentially fractionated with 10,000 MWCO; 5000 MWCO; and 1000 MWCO membranes, and four samples were formed: 0.45μm–10,000Da; 10,000–5000Da; 5000–1000Da; and <1000Da. Mass distribution profiles of COD, 5-day biochemical oxygen demand (BOD 5 ), colour and metals in the aged raw leachate were developed through mass balance. After membrane fractionation of the aged raw leachate, the metals were fractionated with the humic substances. Each fractionated sample as well as the aged raw leachate was oxidised with O 3 only and O 3 /H 2 O 2 . The H 2 O 2 enhanced the reduction of COD and colour; while, the BOD 5 after O 3 only was always higher than that of O 3 /H 2 O 2 . The addition of H 2 O 2 improved the peak reduction of large molecules, but the effects of H 2 O 2 on the fractions of 10,000–5000Da and 5000–1000Da were likely insignificant, which is in accordance with the COD results. No correlation was found between the BOD 5 increase and the area of new peak formed after oxidation. However, the BOD 5 of each sample after oxidation with O 3 only was the logarithmic function of its total peak area.