Swiss Bogs of National Importance were designated as protected areas in 1991. However, their quality was found to have decreased over a 5–7 year monitoring period. In this article we assess the quality of the vegetation and its changes in 80 bogs throughout Switzerland. To determine change over time, 2912 pairs of records from revisited plots were compared. The indicator for quality was the ratio of specialists to the full species list in each record. We performed linear regressions to assess the proportion of variance explained by the variables: bog area, distance to edge, bog density, altitude and draining structures within the bog core areas and their surroundings. To specify the site conditions, we analysed Landolt’s mean indicator values for light, humidity and nutrients. Finally, we derived a raw estimate of the loss in bog area. Distance to edge, bog density and draining structures outside the bogs had the best explanatory power for the quality of bog vegetation. The quality increased with distance from the edge. It correlated better with nutrients indicator values than with humidity values. With regard to quality changes, humidity indicator values decreased mainly in the bog centres, whereas increasing nutrients values and decreasing light values mainly affected the margins. We estimated that the loss of high quality surface affected about 0.6 % of the total Swiss bog area.