||Oxygen has been used in heating, melting, and refining applications for many years. Until recently, oxygen was used to either improve furnace productivity or to extend furnace campaigns. Due to high energy costs, oxygen enrichment today is gaining acceptance as a method of energy conservation and cost reduction. The use of oxygen in combustion results in higher flame temperature, higher available heat, lower flue losses, lower flue gas volume, increased production rates, and higher energy efficiencies. The three accepted techniques of introducing oxygen into a furnace are: (1) mixing with combustion air, (2) lancing, and (3) with burners that are designed to combust fuel with pure oxygen. The choice of a particular technique depends upon the furnace type, the balance between the capital costs and expected benefits, and operating considerations. The first two options require less capital investment compared to the last, but are somewhat limited in the amount of oxygen they can use efficiently. Establishment of design criteria for oxygen capable burners and evaluation of the use of oxygen under industrial conditions are subjects of an on-going research project sponsored by the Gas Research Institute. This paper reports on the effects of oxygen enrichment on the performance of off-the-shelf air-fuel burners. In the tests, oxygen was mixed with combustion air to achieve oxygen concentrations up to 35%. The tests were conducted under controlled high temperature conditions in a refractory-lined water-cooled chamber at firing rates up to 3.75 MH BTU/hour. The reported data include refractory temperature profiles, gas temperature profiles, burner temperatures, burner pressures, and various 165 gas analyses. The data span the entire operating range of the burners. Many standard air-fuel burners are capable of operating with 25~ oxygen in combustion air. At higher oxygen concentrations, their performance is limited by short flame lengths, high flame temperatures, and the original materials of construction. Conclusions regarding maximum enrichment levels, turndown, emissions, etc., are derived based on the data for each type of air-fuel burner. These are expected to lead to a more efficient use of oxygen enriched air, and to better burner designs capable of using air streams containing high oxygen concentrations.