As the outer layer of trees and shrubs, bark is exposed to the direct action of atmospheric conditions and reacts to changes in relative air humidity. This study focuses on the actual hygroscopicity of the bark, regarded as a component of the total bark retention capability. The main research aims were to: (1) determine the physical properties (specific density, bulk density, total porosity), actual hygroscopicity and maximum water storage capacity of the stem bark at breast height (1.3 m) of eight forest tree species; (2) assess the relationship between bark actual hygroscopicity and its physical properties; (3) determine the share of the actual hygroscopicity of bark in its maximum water storage capacity. Significant differences were observed among the different species considered as a consequence of the variation in physical properties of their bark. Actual hygroscopicity of bark (expressed in balance units), i.e., the maximum water amount that can be absorbed from saturated air by the outer bark layer, showed a significant relationship with bark physical properties. Depending on tree species, actual hygroscopicity may constitute from 10 to 30% of the maximum water storage capacity of bark.