A supported or cast film of a polyhydroxyalkanoate polymer, such as a polyhydroxybutyrate/valerate polymer, can be prepared by applying a layer of molten polymer to a cool (typically 4 to 20 °C) surface not substantially above the glass transition temperature of the polymer, so as to form a solid, glassy film with a high density of nucleation sites; subsequently the temperature of the film so formed is raised, for example to 40 °C or more, towards the optimum temperature for growth of the polymer's spherulites effectively separating the film formation/nucleation step from the crystallisation step (growth of spherulites around the nucleation sites). Smaller crystallites are formed, and the film has improved barrier properties.