Composites based entirely on renewable materials with flax fibers as reinforcement and cellulose acetate butyrate (CAB) as the matrix were prepared by compression molding. Scanning electron microscopy of the fracture surfaces showed insufficient penetration of the matrix into the fiber mat. Rheological measurements indicated that this was caused by the high melt viscosity of CAB. Various amounts of furfuryl alcohol (FA) were added to the matrix to control the melt viscosity of CAB. The melt viscosity was decreased dramatically by the introduction of FA, which acted as a CAB solvent and facilitated the impregnation of the flax fiber mats. The mechanical and dynamic thermal properties of composites based on flax mats and various amounts of CAB and FA were investigated. The addition of FA to CAB and the polymerization of FA resulted in a linearly increased modulus and an increase in the maximum fiber stress (strength) of flax composites but a decreased toughness. Dynamic mechanical thermal analysis (DMTA) showed that CAB/poly(furfuryl alcohol) (PFA) matrices were miscible because the glass-transition temperature (T-g) in the resulting blends occurred between the T-g of the homopolymers. DMTA also showed that increasing the amount of FA in the matrix substantially increased the storage modulus of the composites at temperatures lower than 80degreesC. It was possible to tune the storage properties of the composites through the addition of appropriate amounts of FA to the matrices. The CAB/PFA matrix showed behavior between that of thermoplastics and thermosets because of the miscibility and affinity of its components. (C) 2003 Wiley Periodicals, Inc.