This paper presents the results of a monitoring study to assess the spatial variation of CO concentrations within the Westminster City Council building, an office building close to two busy roads in Central London. In general, CO concentrations and correlations with outdoor CO were found to decrease both horizontally and vertically throughout the building away from the busy roads, while the time delay between outdoor and indoor peaks generally increased. This indicates that, within a building, the protection afforded by the building shell to outdoor pollution may be increased away from busy roads. Additionally, the local meteorological conditions were observed to influence significantly the CO concentrations. In particular, south-easterly winds resulted in the highest indoor and outdoor levels, although the highest I/O ratios were for north-westerly winds. © 2006 Elsevier Ltd. All rights reserved.