A de-trending technique was developed for short-term and annual variations to identify long-term trends in primary and secondary pollutants. With this approach, seasonal and annual variations are removed using a mean year; the residual meteorol. short-term variation is removed using a multiple linear regression model. This method was used to de-trend O3, NOx, volatile org. compd. (VOC) and CO concns. in Switzerland. Primary pollutants (NOx, VOC, CO) at urban and suburban stations exhibited a downward trend over the last decade which correlates well with redns. in estd. Swiss emissions. Despite large decreases achieved in precursor emissions, summer peak O3 concns. did not show any statistically significant trend over the last decade. Applying this method to O3 concns. measured at the Jungfraujoch (3580 m above sea level) also showed no trend over the last 10 yr. De-trended summer O3 correlated well with European Union gross national product and industrial prodn. growth rates. These results suggested that if substantial redns. in summer peak O3 in Switzerland are desired, emissions redn. strategies must be part of control programs involving a much larger region.