Poly(beta-hydroxybutyrate) (PHB) modified with different amounts of pentaerythritol ethoxylate (PEE) has been synthesized using Ralstonia eutropha. The growth kinetics and the synthesis of PHB in the presence of PEE were modeled using appropriate differential equations for the mass balance of the two-stage process. The influence of PEE addition on the morphology of PHB was studied by various microscopic and scattering techniques. Light microscopic and wide-angle X-ray measurements indicated that the addition of PEE had a nucleating effect on the crystallization of PHB. The spherulite growth rate was widely independent of the PEE addition. The lamellae of PHB became more disordered when PEE was added as demonstrated by atomic force microscopy. Furthermore, small-angle X-ray data indicated a decrease in the long period with increasing PEE content of the modified PHB.