Humic substances were isolated from different origin peats of the Tula region: reed fen peat (RFP), sphagnum high-moor peat (SHP), black alder fen peat (BFP), sphagnum transition peat (STP) by water-alkaline extraction. The yield of humic substances from reed fen peat is maximal and comes in at 11,1±0,2%; The minimum yield is from sphagnum transition peat 7,4±0,2%, which is explained by the degree of transformation of the initial biomaterial and the conditions of occurrence of peats. Analyzed peats are in a row according to the content of humic substances: RFP>SHP>BFP>STP. The yields of hymatomelanic acids come in at 23,3–25,3%. Polydispersity of humic substances and monodispersity of hymatomelanic acids have been proved by gel-penetrating chromatography. Four fractions differing in their molecular masses and relative content were obtained from humic substances of peats regardless of their origin. Values of the molecular masses of the fractions of HS of BFP are dominant and 25% higher than of the fractions of HS of RFP. Sphagnum peats have a similar qualitative composition and, as a consequence, have similar molecular masses of isolated humic substances correlating with the degree of peat decomposition and with the yields of humic substances. Hymatomelanic acids are one fraction: the molecular weight of GMAs of BFP is 42 kDa, which is 44% higher than of GMAs STP. The correlation between the degree of humification and peat's genesis was revealed. It was shown that the amount of hymatomelanic acids in humic substances of peats can be marker of phases of humification of peats.