Ultrathin films of biodegradable poly[(R)-3-hydroxybutyrate] (PHB) and its random copolymer poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] (PHBHx) were prepared by spin-coating onto aluminum substrates with a naturally oxidized aluminum oxide (AO) surface layer or, alternatively, on gold substrates. The opposite surface of the film was in contact with ambient air. Isothermal crystallization kinetics of these films at room temperature were studied using infrared reflection absorption spectroscopy. The overall crystallization rate for all the polymers when crystallizing on AO is significantly retarded compared with the same polymer crystallizing on gold. It was found that the retardation effect was not due to a confinement effect. The crystallization retardation effect was especially enhanced for PHBHx with a higher (R)-3-hydroxyhexanoate content. Avrami analysis showed that the crystallization rate constant k (min(-1)) for all of the polymers on AO is approximately 3 to 4 orders of magnitude less than that found for the same polymer on gold. Grazing incident wide-angle X-ray diffraction showed that polymers on gold have both flat-on and edge-on crystallite orientations, whereas polymers on AO have a dominating edge-on crystallite orientation. Infrared studies on a quasi-monolayer film revealed no detectable H-bonding between PHB/PHBHx and the AO surface. The crystallization retardation mechanism was explained as being a sum of the dipole-dipole interactions of -C=O of PHB or PHBHx and the -O-Al-O- groups of AO coupled with the rigid disordered amorphous nature of the AO surface.