The process for producing polyhydroxyalkanoates or PHA from volatile fatty acid (VFA) molecules, referred to as precursors, produced by anaerobic fermentation from fermentable biomass, characterized in that it comprises at least the following steps: a) extracting the volatile fatty acid (VFA) molecules, without stopping the fermentation, via an extraction means chosen from means that are, at least, insoluble in the fermentation medium, b) collecting, outside the fermentation reactor, the volatile fatty acid (VFA) molecules once they have been extracted, c) synthesizing, by halogenation, using a type of volatile fatty acid (VFA) chosen from the volatile fatty acids collected in step b) and defined according to the desired type of PHA, a given α-halo acid, d) synthesizing from this α-halo acid molecules of a given α-hydroxy acid by reaction with a base, e) polymerizing using the α-hydroxy acid obtained a defined polyhydroxyalkanoate (PHA).