Commercial, industrially produced mimosa, quebracho, pine, and pecan poly-flavonoid tannin extracts water solutions of different concentrations were examined by rheometry by measuring dynamic moduli both as a function of strain amplitude and at varying frequency. The water solutions of these materials have been found to behave in general mainly as viscous liquids at the concentrations that are generally used for their main industrial applications. Clear indications of viscoelastic response are also noticeable, among these the crossover of the elastic and viscous moduli curves at the lower concentrations of the range investigated, with some differences being noticeable between each tannin and the others-pine and quebracho tannin extracts showing the more marked viscoelastic behavior. Other than pH dependence (and related structural considerations), the parameters found to be of interest as regards the noticeable viscoelastic behavior of the tannin extracts were the existence in the solutions of labile microstructures that can be broken by applied shear. This is supported by the well-known thixotropic behavior of concentrated, commercial polyflavonoid tannin extracts water solutions. Such microstructures appear to be due (1) to the known colloidal interactions of these materials already reported, or (b) to other types of secondary interactions between tannin oligomers and particularly between tannin and carbohydrate oligomers. The latter is supported by the dependence of this effect from both the average molecular masses of the tannin and of the carbohydrate oligomers.