Abstract: In this work, a pair of group interaction parameters, aCCOH, CHO and aCHO, CCOH, used in the UNIFAC group contribution method, has been supplemented from experimental vapor-liquid equilibrium data of iso-butanol iso-butaldehyde system available from literature[5]. The UNIFAC method has heen used in phase equilibrium calculations for systems of n-and iso-butanol purification process with accuracy approaching or attaining that of results calculated by NRTL and other correlations. In view of the fact that the UNIFAC method is based upon the assumption of additive property of group properties, it is necessary to check the calculating results with a few experimental data and to make appropriate choice of the available group parameters.