Humic-like substances (HULIS) were isolated by a solid-phase extraction procedure from PM2.5 fraction urban-type atmospheric aerosol samples collected in downtown Budapest, Hungary, in the nonheating season. Their amount was derived, and the surface tension properties of their pure solutions were investigated. Carbon content of HULIS accounted for 54% of the water-soluble organic carbon and for 19% of organic carbon in the aerosol. Solution of pure HULIS in concentrations corresponding to real incipient cloud droplets decreased the surface tension of water by about 30%, and the major decrease occurred within several tens of seconds. In diluted solutions, however, the thermodynamic equilibrium in surface tension was only reached after several hours, but the equilibrium depression value was still remarkable, about 18%. Detailed analysis of the relaxation curves implied that the kinetics of the surface tension depression should be regarded as and described by a diffusion-controlled process. The surface tension curves as a function of the HULIS concentration after an elapsed time of up to about 100 s resembled the curves observed earlier, but the shape of and tendency in the equilibrium curve (the isotherm) was rather different. The deviations were linked to physicochemical properties of certain classes of compounds within the HULIS. The surface tension depended on the solution pH as well; the smallest depression was observed at about pH = 5, and it was increased by an additional 13% for strong acidic or basic conditions; hence the role of surface tension in cloud droplet formation under acid rain conditions is further amplified.