The invention relates to a method for separating hemicelluloses from a biomass containing hemicelluloses. The method includes the steps of: a) extracting the hemicelluloses from the biomass containing hemicelluloses by treating them with a complex compound in an aqueous solution, and forming a soluble hemicellulose complex, and b) separating the complexed hemicelluloses from the biomass. The method is especially suitable for producing high-quality chemical conversion pulp from raw cellulose. The products produced with the method of the invention have a high degree of purity, and the method is extremely economical and causes few emissions. Hemicelluloses in a pure form can be obtained as by-products of the method.