A new diacid monomer containing an imide ring and a methoxy pendant group, 3-trimellitimido-4-methoxybenzoic acid TMBA, was synthesized and characterized. The diacid was used to synthesize six novel poly(amide-imide)s, (PAIs), through a polycondensation reaction with different diamines in the presence of triphenylphosphite, calcium chloride, and pyridine in N-methyl-2-pyrrolidinone. The chemical structures of the diacid monomer and poly(amide-imide)s were confirmed by FTIR and 1H NMR spectroscopy. All PAIs were obtained in high yield with inherent viscosities ranging from 0.32 to 0.54 dL g-1. They were soluble in aprotic solvents, and exhibited film forming ability. The PAI films displayed glass transition temperatures between 281 and 377 °C, as determined by dynamic mechanical analysis, and they show high thermal stability. Gas permeability results for TMBA-6FDA film show that this PAI could be an interesting material for polymeric membranes aimed to CO2 separation process.