A new one-dimensional thallium(I) coordination polymer with a Schiff base ligand, {[Tl(L)(HL)](H2O)0.77}n (1) [HL=4-hydroxybenzylidene-4-aminobenzoic acid], has been synthesized and characterized. The single-crystal X-ray data of compound 1 show the coordination number of the TlI ions to be seven. The thermal stability of 1 was studied by thermal gravimetric (TG) and differential thermal analyses (DTA). The overall stoichiometry in the solid state is 1:2, but both spectrophotometric and conductometric methods did not support formation of a complex with this stoichiometry in solution. The solvatochromic behaviour of 4-hydroxybenzylidene-4-aminobenzoic acid was also investigated by studying its spectra in a selection of different organic solvents. The observed bands are assigned to electronic transitions.