Annual mean sulphur dioxide concentrations measured at County Hall in central London between 1931 and 1985 showed no clear trend until 1963/64, when a steady decline set in. Results for a number of sites across London for the period 1964/65 to 1984/85 have been divided into three geographic zones: the City of London, a central zone and an outer zone. A steady downward trend is evident in each zone, but is most pronounced in the City of London, especially after 1970. This could reflect the successful introduction of a 1% limit on the sulphur content of oil used within the City, but not elsewhere in London. Annual mean sulphur dioxide concentrations are now around 40 microg m(-3) in all three zones. Highest daily sulphur dioxide levels in both summer and winter have also declined steadily during the period 1964/65 to 1984/85, while at the same time the seasonal pattern of higher winter peaks in daily maxima, which was strong at the beginning of the period, has been virtually eliminated.