Commercial xanthan and xanthan from Xanthomonas strain 646 produced in the laboratory have been subjected to heat treatment for various periods of time in oxygen depleted, high salinity, aqueous solutions. Both the viscosity and the carbohydrate content decreased with increasing incubation time at a specified temperature. The losses increased with increasing temperature. Data from electron micrographs and dialysable sugar content indicate that random cleavage of the double-stranded xanthan chain is the main mechanism responsible for the decreasing viscosity. Removal of pyruvate and acetate substituents on the side chains was apparently not related to the change in physical dimensions. The mannose/glucose ratio in the non-diffusible fraction decreased with incubation time, apparently not related to change in physical dimensions. Electron micrographs showed that one of the samples appeared as highly aggregated in the native condition. After 1 month at 80 degrees C, we observed that the aggregates had dissolved and that the viscosity had increased fivefold. This suggests that heat treatment can be used to avoid microgels and to obtain higher viscosifying power of the native xanthan.