The aim of the present paper is to define the importance of the Py-GC/MS method in the study of structural composition of HAs of forest soils depending on plant litter properties (oak, spruce, thuja). Forest soil sampled from the area of Rogow Forest Experiment Department, Warsaw Agricultural University (WAU), was used for our research. The forest soil (Haplic Luvisols) was sampled in 5 replications under tree stands of: oak (Quercus robur L.), spruce (Picea abies L. Karsten) and thuja (Thuja plicata D.Don.ex. Lamb.). Humic acids (HAs) were isolated from the soil sample using the IHSS method. For the humic acids extracted from horizons Ol, A and E (AE), the spectrophotometric analyses in the UV-VIS range were performed and the elemental composition was assayed. The HA chemical composition was evaluated with the use of pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The humic acids of Ol horizon, irrespective of the tree stand, showed higher H/C values and lower O/H values as compared with HAs from A and/or E (AE) horizons. Humic acids pyrolysis products were identified as aromatic single- and multi-ring compounds, compounds of lignin origin, phenolic compounds, polysaccharide compounds, and nitrogen-containing compounds as well as aliphatic compounds. The dominant compounds, being part of humic acids pyrolysis products, were single-ring aromatic hydrocarbons, which accounted for 58.81 to 76.20% of all the compounds identified. Generally it should be stated that the share of particular groups of compounds varied with soil depth and depended on the species of tree.