Aitken mode atmospheric particles largely control the formation and transformation of cloud systems, which in turn have an effect on their radiative properties. In this paper, a measurement technique for the cloud condensation nuclei (CCN) activity of monodisperse particles is described and the measurement data (results) obtained for monodisperse aerosol fractions with diameters of 60, 80, and 100 nm are presented. The measurements were performed during March and April 2014 at the urban background station located in a suburban area of St. Petersburg. The CCN concentration in the water-vapor supersaturation range of 0.1–1.1% and the cloud particle activation parameters were determined. The fraction of active particles capable to serve as CCN is found to be less than 33%, which is characteristic of anthropogenic aerosols containing hydrophobic organic components. Based on the measurement data, the hygroscopicity parameter κ characterizing the chemical composition effect on condensational properties of particles has been determined. During the measurement campaign, the value of the hygroscopicity parameter varied between 0.1 and 0.8. The lower limit of κ corresponds to organic species and its higher values occurs due to presence of hygroscopically active inorganic species in particles. The average value of κ was estimated to be 0.34 ± 0.19, which is in good agreement with literature data obtained for megapolises and European continental aerosols.