The complex formation equilibria between gadolinium(III) ion and moxifloxacin (MOXI) were studied in aqueous solutions. The investigations were performed by glass electrode potentiometric (ionic medium: 0.1mol dm-3 LiCl, 298 K) and UV spectrophotometric measurements. In the concentration range 0.5≤ [Gd3+] ≤ 1.0; 1.0 ≤ [MOXI] £ 2.0 mmol dm-3 ([MOXI]/[Gd] = 1: 1to 5: 1) and pH between 2.5and 9.0, gadolinium(III) and moxifloxacin form the complexes of the composition: Gd(HMOXI)3+, Gd(HMOXI)23+, Gd(HMOXI)33+, Gd(HMOXI)2MOXI2+, Gd(HMOXI)(MOXI)2+, Gd(MOXI)3. The stability constants of the complexes were calculated with the aid of Hyperquad2006 suite of programs, taking into account the hydrolysis of Gd3+ ion and protonation of moxifloxacin anion. The possible structure of the complexes, in solution, and their formation mechanism is suggested. The effect of moxifloxacin, and for comparison purpose, DTPA on gadolinium(III) plasma speciation was evaluated by computer simulation.