The phosphine derivati-ves having general formulae; R P, R RP and R PX 322(where R = aryl or substituted aryl group, R = alkyl or alkoxy group and X = halogen) react with iodine to form 2:1, 1:1 and 1:2 addition compounds in solution. The solvent acetonitrile, exerts an ionising affect on the adducts formed. The interactions are s tudi ed by conductometri c and spectroscopi c methods. In presence of excess of phosphine derivative concentration, forma­tion of iodide ions takes place.2 Rip + 12 (R2RP)2I2 (R2RP)2I+ + I and R2RP + 12 R/PI2 R2RPI+ + I . The reaction involves the formation of an equilibrium system predomi­nantly lying towards the right side. When excess of iodine is present, tri­iodide ions are formed: The spectroscopic results complement the results obtained by conductometric measurements.