Abstract Composites of an aliphatic polyester (Bionolle) with natural flax fibres are prepared by batch mixing. The effect of processing conditions on fibre length distribution and the dependence of the composite mechanical properties on fibre content are investigated. The tensile modulus changes with fibre content according to the modified rule-of-mixture equation, with a fibre orientation efficiency factor η 0 =0.194. The strength of Bionolle/flax composites tends to decrease with fibre loading, showing that there is no adhesion between matrix and fibres. With the aim to improve fibre–matrix adhesion, surface chemically modified flax fibres are also tested as reinforcing agents. A 30% strength increase is observed when natural fibres (25 vol%) are substituted by fibres containing acetate groups. No significant strength changes are observed in composites containing fibres with valerate groups or polyethylene glycol chains grafted at the surface.