The characterization of polyhydroxyalkanciates (PHA) produced by mixed cultures is fundamental for foreseeing the possible final applications of the polymer. In this study PHA produced under aerobic dynamic feeding (ADF) conditions are characterized. The PHA produced shows a stable average molecular weight ((M) over bar (w)), in the range (1.0-3.0) x 10(6), along three years of reactor operation. Attempts to improve the amount of PHA produced did not introduce significant variations on the values of (M) over barw. Along this period, the polydispersity indices (PDI) were between 1.3 and 2.2. The use of different carbon sources allowed the tailoring of polymer composition: homopolymers of poly(3-hydroxybutyrate), P(3HB), were obtained with acetate and butyrate, whereas a mixture of acetate and propionate, and propionate and valerate, gave terpolymers of 3-hydroxybutyrate (3HB), 3-hydroxyvalerate (3HV), and 2-methyl-3-hydroxyvalerate (2M3HV). All of these PHA had (M) over bar (w) between 2.0 x 10(6) and 3.0 x 10(6) Thermal characterization of the produced polymers showed values of glass transition temperature, melting temperature, melting enthalpy, and crystallinity slightly lower than those obtained for PHA from pure cultures. The introduction of a purification step during the polymer extraction process allowed the elimination of possible contaminants but did not significantly improve the polymer quality.