The free tropospheric concentrations of several light hydrocarbons and ozone have been measured throughout most of the year in air flows approaching the UK from unpolluted areas of the Northern Hemisphere. The measurements were made using an aircraft at altitudes between 1500 and 3000 m and concurrent measurements of halocarbons confirmed that the air had not passed recently over potential sources of hydrocarbons. Most of the hydrocarbons show a clear seasonal pattern with a summer minimum and winter maximum, but propene and ethene, which are very short‐lived in the atmosphere and have oceanic sources, show no seasonal pattern. The halocarbon concentrations also exhibit no seasonal pattern but ozone shows a spring maximum. The results for ethane and propane, which have a common anthropogenic source, are consistent with a summer to winter average OH radical concentration ratio of about 4 to 7 and a travel time for source area to sampling point of about 20 days. It is suggested that the presence of large concentrations of non‐methane hydrocarbons on an extensive scale will influence the seasonal pattern of ozone observed in large parts of the northern hemisphere.