This work concerns the local air ratio in a circulating fluidised bed (CFB) furnace, estimated by fluctuating signals from zirconia cell probes and compared to simultaneous analysis of concentrations of extracted gas samples. The time fraction during which the fluctuating zirconia cell signal shows oxidising gas conditions is strongly correlated to the local air ratio of the gas. This correlation is characterised by two parameters that depend on the fuel properties and the position in the furnace. When the two parameters are determined, the fluctuating signals from zirconia cell sensors can be used to obtain the air ratio at different heights in the furnace. This provides a cheap and robust technique for on-line monitoring of the gas conditions in the furnace when, for example, optimising the operation of a CFB boiler to reduce nitrous oxide emissions. The method is limited to the freeboard, because the vivid fluctuations of gas concentrations in the bottom bed region are faster than the response time of this type of sensor. (C) 2003 Elsevier B.V. All rights reserved.