A brief description of air pollution and its significance to conservation is followed by an assessment of four methods of measuring gaseous air pollution within museums. Levels of sulfur dioxide, nitrogen dioxide, and ammonia were measured at the Tate Gallery, London, levels of sulfur dioxide were measured at the National Gallery of Ireland and the Victoria and Albert Museum, London. Results indicate significant amounts of pollutants entering the museums. Levels varied considerably according to weather conditions but it was possible to identify consistent variations between particular galleries. Activated carbon filters in air-conditioned galleries were effective in virtually eliminating sulfur dioxide. Enclosure of works of art and reduction of the airflow around painting canvases were effective in reducing the penetration of air pollution.