The effects of the heat and drought of the summer 2003 on carbon (C) fluxes (soil CO₂ efflux, dissolved organic carbon in soil, C from litterfall and C sequestration in tree) were investigated in three mature forests in Switzerland between 2001 and 2005. Measurements of the soil matric potential (W) indicated a gradient in drought intensity at the three sites (Vordemwald, Schänis, Beatenberg) with average W during the summer 2003 of -75, -41 and -5 kPa, respectively. In Vordemwald where the drought was the most severe, the hourly soil CO₂ efflux declined by about 30% when W < -90 kPa. Estimated annual soil CO₂ efflux in 2003 was about 10% smaller than effluxes in wetter years (2001, 2002 and 2004) but the differences were not significant. In Schänis and Beatenberg,no suppression of hourly soil CO² efflux was observed during the summer 2003. The annual efflux in 2003 at both sites was 10–15% larger than the effluxes in the other years but the differences were not significant. DOC concentrations increased during the first rewetting events after the drought in Vordemwald and in Schänis. This was mainly due to a concentration effect due to the low soil water content. Because precipitation was lower in 2003, annual DOC fluxes substantially decreased at the three sites. Carbon sequestration in tree stems in Vordemwald declined by 32% in 2003 compared to the mean 2002–2005. In Beatenberg and Schänis, tree growth responded little to the extreme summer 2003. Litterfall at the three sites showed no marked reaction to the summer 2003. Estimate of annual net ecosystem production (NEP) suggested that the stands in Vordemwald and in Schänis remained a C sink between 2002 and 2005 but that the extreme summer 2003 caused a decrease in NEP. In Beatenberg, the impact on the NEP seemed to be limited in 2003. The annual NEP in 2003 decreased with increasing drought intensity over the three sites.