The variation in carbon monoxide (CO) mixing ratio is of primary interest since it affects the atmospheric abundance of several trace gases, like methane, via extensive chemical feedback mechanisms. From the mid-1990s the global annual average mixing ratio of CO varied by about ± 0-1% yr-1 while in 1998 a great pulse of CO emission increased the global CO concentration through mid-1999 by 16% over the background level. This surplus was primarily caused by heavy forest fires in several regions of the world. Our study was performed to reveal the potential causes of changes in the European CO budget and mixing ratio between 1995 and 2000 using a simple tropospheric box model. The primary anthropogenic emission from fossil fuel combustion and industrial processes in Europe showed a fairly constant annual decrease of about 4%yr-1 between 1995 and 2000, while the secondary anthropogenic CO source strength was reduced at a rate of about 3%yr-1. This decrease was partly balanced by sporadic pulses of European forest fire emissions. In spite of the reduction in the sources the region remained a net CO producer. The incremental change in the global CO budget caused by European sources was about 47.2 ± 15.2 Tg CO yr-1 on average. The output of the model was validated against available observational data, both in absolute terms and with respect to the seasonal variation of the CO mixing ratio. The model revealed that the downward tendency of CO mixing ratios in Europe was temporarily offset by the contribution of large-scale forest fires in the northern hemisphere through long-range atmospheric transport. © 2005 Elsevier Ltd. All rights reserved.