Ternary mutual diffusion coefficients (D11, D22, D12 and D21) for aqueous solutions containing nickel chloride and theophylline (THP), at T=298.15K, and at different carrier concentrations were measured by using the Taylor technique. The results are compared with those obtained for the binary systems H2O–NiCl2 and H2O–theophylline, already published. These data permit us to have a better understanding of the structure of this ternary system and its thermodynamic behaviour. For example, it is possible to take conclusions about the influence of this electrolyte in diffusion of THP, and to estimate some parameters, such as the number of moles of each component transported per mole of the other component driven by its own concentration gradient.