The association of positively charged copolymers based on N-isopropylacrylamide with a hydrophobically modified poly(sodium acrylate) derivative, containing 3 mol % octadecyl groups, PANa3C18, is investigated. The NIPAM-based copolymers, PNIPAMx, contain a fraction x of a positively charged comonomer, 0 less than or equal to x less than or equal to 25 mol %. In PANa3C18/PNIPAMx aqueous mixtures the formation of mixed octadecyl/PNIPAM hydrophobic aggregates is strongly reinforced by the electrostatic attractions between the two polymers. leading to a pronounced viscosity increase. The maximum viscosity enhancement as well as phase separation phenomena are observed at a roughly constant polymer charge ratio, regardless of x. For PANa3C18/PNIPAM5 mixtures an important thermothickening behavior is observed. but this thermothickening ability weakens as x increases. The gradual loss of thermosensitivity with increasing x is also supported by fluorescence probing experiments and phase separation studies.