The water vapour and carbon dioxide transport properties have been measured of bacterial (isotactic) poly(3-hydroxybutyrate) (iPHB) and its blends with synthetic (atactic) PHB (aPHB) and poly(epichlorohydrin) (PECH) have been measured. The purpose of this work is to study the transport properties of iPHB blends with a miscible second polymer, intended as a polymeric toughener that does not impair the original iPHB biodegradability. Thus, only iPHB rich blends containing up to 40% of aPHB or PECH were considered. The effect of aPHB on the blends is to increase the sorption and permeability with respect to pure iPHB due to the reduction in the level of crystallinity, while PECH tends to decrease the water and carbon dioxide solubilities of the blend. The blends have essentially the same permeability to CO2, although the water transmission rates are lower for the iPHB/PECH system.