The degradation process of linear water-soluble vinyl polymers was followed in aqueous medium at temperatures higher than 150-degrees-C. We found that, for any chemical structure, thermal oxidative degradation of the longest chains always occurs, leading to moieties of lower molecular weight (20,000-60,000). The behavior of swollen tridimensionnal networks was tested under the same conditions, and it appeared that rheological and fluid loss properties of the system could be maintained in that case for an optimum number of crosslinks. The corresponding gels were used in simple formulations containing some other drilling fluid components. The first results concerning the thermal stability of the latter are very encouraging.