the invention relates to an enzymatic - chemical method for obtaining polyhydroxyalkanoates , especially polyhydroxybutyrate , or the copolymers thereof , from biomass . the inventive method comprises chemically treating the biomass with a reducing agent that reduces the non - pha cell components of the biomass . the chemical treatment is carried out before and / or after enzymatic cell disruption . the inventive method allows , unlike other cell disruption techniques , for obtaining polyhydroxyalkanoates from biomass with a relatively low pha content without drastically changing or deteriorating the polymer properties or polymer purity .