The analytical task of determining the phenolic compound content of water-ethanol extracts of Populus tremula L. (common aspen) leaves is complicated by the heterogeneity of compound groups having different polarities and appearing in varying concentrations. The purpose of the present work is to study the conditions of solid-phase extraction and high-performance liquid chromatography used to analyse the content of different groups of phenolic compounds in water-ethanol extracts of leaves from the P . tremula plant. In order to facilitate the derivation of phenolic compounds, an exhaustive extraction process was carried out using ethanol. Solid-phase extraction was carried out using a Diapak C16 cartridge, after which the eluates were passed through a membrane filter having a pore diameter of 0.45 μm. The high-performance liquid chromatography method was used to determine the content of phenolic acids and flavonoid glycosides, as well as salicin and individual flavonoid glycoside components: hyperoside, rutin, astragalin and two unidentified flavonoid glycosides in aqueous (analyte 1) and aqueous-alcoholic fractions (analyte 2) in two systems along the gradient elution. The requirement of analysing the primary aqueous eluate together or in parallel with the main aqueous-alcoholic fraction in the preparation of P. tremula leaf extracts for high-performance liquid chromatography using solid-phase extraction cartridges was substantiated. For separating the extract to determine the hydroxycinnamic and hydroxybenzoic acid content, it is preferable to use system 2; for determining the phenologlycoside (salicin) content, system 1 is more effective. Flavonoid glycosides (hyperoside, rutin, astragalin and two unidentified flavonoids) make the most significant contribution to the difference between the aqueous and aqueous-alcoholic fractions.