A fed-batch method for converting toxic substrates into poly-3-hydroxybutyrate is presented. The method involves a series of batch-growth processes, regulated by adding small amounts of carbon substrate, during the course of which the concentration of the nitrogen source decreases and controls the distribution of the substrate-carbon assimilated. The addition of carbon substrate is controlled, and the small changes that occur in the growth pattern are interpreted using high-resolution reaction calorimetry. The method was tested with Ralstonia eutropha DSM 4058 growing on phenol, and Variovorax paradoxus DSM 4065 growing on sodium benzoate. The maximum carbon conversion efficiencies (CCEs) obtained, 23% and 27% respectively,were compared with the theoretically possible values.