An actual, exchange as well as a total potential (hydrolytic) acidity of litter in bog birch forests was studied at the gradient from tall grass to sphagnum- dead cover forest types within ecological profile which was established transversely to the river-bed of an intrabog stream. It was found that all the kinds of acidity are closely correlated. Litters are characterized by acid and strongly acid reaction of environment by pH 3,7- 5,5 value. The analysis of interaction of acidic litter properties, of group and fractional composition of humus substances as well as the main mineral elements was performed. A close positive relation of litter acidity with fulvic acids of the 3-rd fraction and a negative relation with the sum of humic and fulvic acids of the 1-st fraction as well as with ash level, calcic and ferric oxides were found in its composition. Calcium deficit is the main factor of litter acidity: only calcium oxide is included in prognostic signs by the step-by-step algorithm of multiple regression. The main role in litter enrichment by mineral elements is played by ground vegetation which biogeochemical selection proceeds owing to succession of forest types under changing moistening. Leaf debris forms only a basic background. Thus, moisture of an edaphon which, mainly, defines the composition of ground vegetation in bog birch forests is, indirectly, as an initial source of formation of acidic litter properties.