A statistical analysis of roadside concentrations of nitrogen oxides (NO$_x$) and nitrogen dioxide (NO$_2$) in London shows that from 1997 to 2003 there has been a statistically significant downward trend (at the p 0:004 level) in NO$_x$ averaged across a network of 36 sites. Conversely, there has been no statistically significant trend in the concentrations of NO$_2$ over the same period. Hourly modelling using a simple constrained chemical model shows that the NO$_2$/NO$_x$ emissions ratio from road traffic has increased markedly from a mean of about 5–6 vol% in 1997 to about 17 vol% in 2003. Calculations show that if the NO$_2$/NO$_x$ emissions ratio had remained the same as that towards the beginning of each time series, 14 out of the 36 sites would have shown a statistically significant downward trend in NO$_2$ at the p 0:10 level compared with only five that did. The increase in the NO$_2$/NO$_x$ emissions ratio from road traffic in recent years has therefore had a significant effect on recent trends in roadside NO$_2$ concentrations. It is shown that the increased use of certain types of diesel particulate filters fitted to buses is likely to have made an important contribution to the increasing trends in the NO$_2$/NO$_x$ emissions ratio. However, it is unlikely that these filters account for all of the observed increase and other effects could be important, such as the increased penetration of diesel cars in the passenger car fleet and new light- and heavy-duty engine technologies and management approaches.