Complexation of Eu(III) by two hydroxybenzoic acids, namely p-hydroxybenzoic acid (4-dihydroxybenzoic, HPhbH), and protocatechuic acid (3,4-dihydroxybenzoic, HProtoH 2 ), is studied by time-resolved luminescence spectroscopy (TRLS) in mildly acidic solution.Comparable formation constants are determined at 0.1 mol.L -1 NaCl for EuPhbH 2+log 10 β°(EuPhbH 2+ ) = 2.18  0.09 (1)and 0.01 mol.L -1 NaCl for EuProtoH 2 2+log 10 β°(EuProtoH 2 2+ ) = 2.72  0.07 (1). The stoichiometry and carboxylate complexation of the EuProtoH 2 2+ complex is ascertained by varying both pH and ligand concentration. The luminescence decay time of EuPhbH 2+ (τ = 107 ± 5 µs) is comparable with that of Eu(H 2 O) n 3+(τ = 110 ± 3 µs), suggesting that luminescence quenching processes compensate the expected increase in decay time due to the dehydration associated with complexation. For EuProtoH 2 2+ , the luminescence decay time is even shorter (τ = 20 ± 5 µs), evidencing intricate quenching processes.

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