[en] The interactions of uranyl ion with phthalic acid and 3-chloro, 3-bromo, 3-iodo, 3-nitro, 4-nitro and 3:6-dichloro phthalic acids have been investigated by the Calvin-Bjerrum titration technique. The stability constants of these complexes have been determined at 30⁰C at 0.1 M (NaClO₄) ionic strength. Uranyl ion forms 1:1 complexes with all ligands, while a 1:2 complex is formed only with phthalic acid. All the 1:1 uranyl complexes except those of 3:6-dichloro and 3-iodo phthalic acids follow the relationship log K = apk + b. The deviation of the two chelates from the linear relationship is explained on the basis of steric effects and L → M π-bonding. (author)