In the months of May and June, 2008, about 20 deaths from CO poisoning were reported in Nigeria. Ignorance along with lack of information and data about this toxic substance can be blamed for these deaths. In this study, a preliminary attempt is made to evaluate the diurnal trend in CO generation and distribution at several road junctions and motor parks in Benin City. A dosimeter (in situ method) that uses an electrochemical sensor to measure ambient levels of CO was used. At the 5 sampling locations selected, very high CO concentrations were measured with a mean range of 14.8-28.3 ppm. The 10.0 ppm statutory limit set by the Federal Ministry of the Environment, Housing and Urban Development (FMEH&UD) was clearly exceeded. Diurnal variations in the data were statistically significant (P < 0.05), with the highest CO concentrations recorded in the morning hours. Spatial variations were also statistically significant, with the highest mean CO load of 28.3 ppm measured at Sokponba road junction. Vehicular exhaust was identified as the main CO source in the city. Frequent traffic jams resulting from poorly maintained roads, high traffic density, unfavourable traffic handling, inadequate traffic discipline and very low wind speed are identified as the main factors responsible for the high emissions, accumulation, and low dilution and dispersion of the generated CO.