Complex formation between aluminum(III) ion and fluoroquinolone antibacterials-either moxifloxacin (4th generation antibiotic) or fleroxacin (2nd generation antibiotic) were studied in aqueous solutions without and in the presence of sodium dodecylsulfate (SDS). The investigations were performed by glass electrode potentiometric (ionic medium: 0.1 mol/dm(3) LiCl, 298 K), UV spectrophotometric, multinuclear (1H and 13C) magnetic resonance and ESI-MS measurements. The experimental data were consistent with the formation of Al(HL)L2+, Al(HL)3+ AlL2+, Al(OH)L+ and Al(OH)2L complexes in the pH interval ca. 3-8 and up to 5 : 1 ligand to metal mole ratio with range of Al3+ concentrations between ca. 0.025 to 1.0 mmol/dm3. The binary complex, AlL2+ is fairly stable (log beta(1,0,1) ca. 11.0) and its stability increases in the presence of SDS. At higher concentration ratios of ligands to aluminum, up to 5 : 1, the complex Al(HL)L2+ is formed with rather high overall stability constant (log beta(1,1,2) ca. 24.0). The ESI-MS data generally, confirmed the derived model, and the formation of the complex with ligand to metal ratio 2 : 1. NMR measurements indicate that both ligands utilize 4-carbonyl and carboxyl oxygens as donor atoms. The presence of surface active substance, SDS, favors the formation of the complex in which the ligand is protonated, i.e. Al(HL) and its maximum formation is shifted toward milder acidic region (pH ca. 4). The aluminum-quinolone complexes may affect the bio-distribution of both, quinolone and/or aluminum ion upon concomitant ingestion of aluminum-based antacids or phosphate binders and fluoroquinolones.