The Roundwood 11`` Mill, Rotherham Engineering Steels Ltd., produces bar by re-heating steel billets in a walking beam type furnace with a designed continuous throughput capacity of 110 t/h. The furnace has six independently controlled zones and is fired by interruptible natural gas, with heavy fuel oil used as an alternative. To improve the efficiency of fuel use, five zirconia-based oxygen measuring probes, have been installed in the roof of the furnace. These continuously monitor the oxygen concentration of the combustion products in each of the upper furnace zones. In the heating zone the measured value is used to trim the air/fuel ratio for the burners to produce the desired oxygen level. This report presents the results of an independent assessment of the air/fuel ratio control technique in terms of furnace energy use and operating procedures compared with the conventional method of control. The total cost of the project was 26,572 pounds. An annual fuel saving of 15,300 GJ has been realised, with a total reduction in operating costs, including reduced maintenance requirements, of 31,000 pounds/year. This equates to a recovery of the project costs in about 10 months. (author).