During an intensive field campaign at a midlevel (1640 m above sea level) mountain in central Europe, cloud water samples were collected using a cloud water sampler and subsequently analyzed for organic and inorganic ions, total carbon (TC), and black carbon (BC). Cloud water surface tension (σ) was measured using the ring method. Generally, the cloud water samples had surface tensions lower than that of pure water. The average value of σ was 95.2% of the σ of water, while the lowest value was 83.3%. A trend of lower σ with higher concentrations of organic material (OC, obtained as TC-BC) was found at lower OC concentrations than those previously described <Charlson et al., 2001; Facchini et al., 1999a>. The effect of the lower σ on cloud droplet activation was modeled using K¿hler theory. For an assumed dry sulphate nucleus with 0.05 ¿m diameter, the critical supersaturation would be decreased by 7% (average σ) and by 22% for minimum σ.