EPR simulation method together with pH-potentiometry combined with UV–Vis spectrophotometry were used for the study of the ternary system 4-fuorosalicylic acid (HA)–N,N-diethylnicotinamide (B)–copper(II) in aqueous solution. The N,N-diethylnicotinamide ligand is a weak donor, its mixed–ligand complexes with 4-fluorosalicylate anions are more favoured. The number of coordinated N,N-diethylnicotinamide molecules increases with decreasing temperature: up to four ones were detected in the coordination sphere of copper(II) in frozen solutions. The formation of [CuH −1 AB 2 ] and [CuH −1 A] was detected by all methods at neutral pH. At lower pH values, [CuA 2 B 2 ] and [CuB] become dominant, and this fact is in good agreement with [CuA 2 B 2 (H 2 O) 2 ] crystals obtained from similar solutions. The structural unit of the [CuA 2 B 2 (H 2 O) 2 ] complex consists of a copper(II) ion, which is monodentately coordinated by a pair of 4-fluorosalicylate anions and by a pair of N,N-diethylnicotinamide in trans positions in the basal plane, and by two water molecules in the axial positions of a tetragonal bipyramid.