The regularities of the prehydrolysis of biomass for leaf bearing (oak) and coniferous (pine) wood and control tests with pure cellulose were studied. It has been shown that the quality of the composition of liquefaction products changes depending on the content of pentosanes (xylans), Optimal conditions of the prehydrolysis of biomass have been established in the presence of solvents (ethylene- and propylene-glycol) with water-alkaline impregnation. Formed as a result of the prehydrolysis, depolymerization of the carbohydrates (monomeric sugar, levoglucosan and oligosugar) were identified by their solubility in water. Further pyrolysis of hydrolysates proceeded through stages: - dehydration of the carbohydrates in the interval of temperature 280-350degreesC. chemical transformations of hydroxycarbonyl compounds (350-500degreesC), mutual transformations of aldose to ketose (De Bruin and Van Ekenstein reactions), tautomeric transformations (changes oxo --> cyclo) and stabilization of products as simple ether. The activation energy of both stages of decay have been rated and a sufficiently grounded hypothesis has been suggested for formation of the products of various kinds of wood liquefaction.