A poly(hydroxybutyrate-co-hydroxyvalerate) with 9% hydroxyvalerate content has been thermally degraded to give a set of materials of different molecular weights. The effect of molecular weight on the lamellar structure, thermal and mechanical properties was investigated. The long period, lamellar and amorphous thickness all increase as molecular weight increases; their values vary linearly with 1/(molecular weight). Observed melting temperatures increase with molecular weight, following the same functional form, while melting enthalpy and non-isothermal crystallization temperature decrease. Young's modulus varies by 13% with molecular weight; changes in crystallinity cannot explain this effect in detail. Ultimate tensile strength increases rapidly with molecular weight and then levels off at 28.5 MPa above 10(5) g/ mol. This can also be seen as a linear variation with 1/(molecular weight). The strain at the point of ultimate tensile strength also increases rapidly up to 10(5) g/mol but then continues to increase at a slower rate.