An atmospheric HULIS (humic-like substance) sample dissolved in water was used to generate particles with different dry diameters. A HHTDMA (High Humidity Tandem Differential Mobility Analyzer) and LACIS (Leipzig Aerosol Cloud Interaction Simulator) were used to measure hygroscopic growth of the HULIS particles. LACIS also was used to measure the critical super-saturation for the activation of HULIS particles with dry diameters of 50, 75, 100, and 125 nm. Simple K¿hler theory was used to simulate the measured hygroscopic growth factors. For this, a new technique was used, where the ionic density (ρ ion ) was defined as a combination of the HULIS physical properties for which values could not be reliably determined. By adjusting ρ ion in the K¿hler equation, modeled hygroscopic growth could be brought into agreement with the measurements, even without the explicit knowledge of the different HULIS properties. It was demonstrated that the values of ρ ion determined with our procedure can be reproduced from combinations of physically realistic values of the physical properties represented by ρ ion . Adjustments of the ionic density were done for two different surface tensions, that of water as the upper limit, and the lowest value that had been measured for this HULIS sample (published previously) as the lower limit. The two adjusted values of ρ ion were used in the K¿hler model to derive critical super-saturations. For more dilute droplets, measured and modeled critical super-saturations were in agreement for both values of the surface tension, whereas for the less dilute solutions, agreement only could be achieved when a lowering of the surface tension due to HULIS was taken into account.