The 1 : 1 complex formations between pyridine bases and Eu (DPM)₃ are approved at equimolar, excess base and excess Eu (DPM)₃ conditions. The equilibrium constants and complex shifts are estimated by the graphic and simulation methods of the observed results of the above three conditions. The complex shifts thus obtained showed good correspondence with infinite concent-ration shifts and the so-called S values. The equilibrium constants are solvent dependent, whereas the complex shifts are not.