Thermoassociative graft copolymers, poly(sodium acrylate)-graft-poly(N-isopropylacrylamide), PAA-g-PNIPA, were studied in aqueous solution, with various added co-solutes such as salt, neutral species and anionic surfactant. These additives induced important modifications in the characteristics of thermothickening phenomenon, which were compared with the solubility behaviour of the PNIPA precursor under the same conditions. For all the co-solutes used, we evidenced a strong correlation between the cloud point of the PNIPA/water/co-solute ternary system and the temperature of association of the PAA-g-PNIPA solubilised in the same medium. Fluorescence experiments were used to clarify the precise mechanism of co-solute effect: "solvent perturbation" or "polymer binding". Potassium carbonate and glucose were shown to lower the quality of solvent for the PNIPA side chains so that the thermothickening phenomenon was shifted to lower temperatures. On the contrary, sodium dodecylsulphate and hexanol interact with PNIPA grafts through the formation of hydrophobic aggregates.