In various industrial fields, water soluble polymers are commonly used as thickening agents to control the reology of aqueous fluids. Nevertheless, their properties are weakened as the temperature increases. In order to overcome this problem, the concept of ＇＇thermoassociative＇＇ water soluble polymers was developed. Such new amphilic systems can be obtained by grafting on an hydrophilic backbone, side chains which become non water soluble above a Lower Critical Solution Temperature (LCST). Semidilute solutions of these copolymers present reversible thickening properties as the temperature increases and reaches a critical value, close to the side chains LCST. This behaviour can be related to the agregation of the grafts above their LCST, into ＇＇hydrophobic microdomains＇＇, inducing the formation of a three dimensional network. At higher temperature, the viscosity of the solution sheared at a constant rate reaches a maximum value. This can be interpreted in terms of the reorganisation of the physical network under shear, from a structure with mainly ＇＇intermolecular associations＇＇ to a system with an increasing number of ＇＇intramolecular associations＇＇. Owing to the diversity of water soluble polymers exhibiting a phase separation on heating (LCST), different ＇＇thermoassociative＇＇ copolymers were realized [1] and [2], using either polyelectrolyte or neutral hydrophilic backbone. In the aim of applications of such systems in the oilfield industry, copolymers containing 2-acrylamido 2-methyl propane sulfonic acid (AMPS) were developed, using polyethylene oxide (PEG) as LCST grafts. A general description of their rheological behaviour will be given here. Their properties can be controlled either by varying the solution characteristic (polymer concentration, salinity, etc.) or by modifying the chemical structure of the copolymer (grafting ratio, molecular weight of the backbone, etc.). This rheological study showed the potentiality of the ＇＇thermoassociative＇＇ system, particularly towards the oilfield industry [3]. It is now possible to have thin cement slurries at ambient temperature and still viscous slurries at bottom hole temperature, as desired. The reversibility of the phenomenon joint to the shear thinning properties of the polymer solution are adding a plus for cement slurries and drilling fluids designs. The thermoassociative phenomenon was also studied by Small Angle Neutrons Scattering (SANS, using copolymers composed by a deuterated polyacrylate backbone and protonated polyethylene oxide grafts. The scattering properties of our system were studied by varying the temperature, the salinity and the polymer concentration of the solution. A good agreement between the microscopic characteristics of the solutions and their rheological properties was found.