The dielectric behavior of polar liquids like isobutanol, isobutyraldehyde and their equimolar binary mixture in nonpolar solvent benzene is studied in the microwave frequency range using the cavity perturbation technique at 6.218 GHz (J band), 9.880 GHz (X band), 16.331 GHz (P band) and 24.951 GHz (K band). Ab initio geometry optimization is performed in 6-31 G (d) basis set using Gaussian 94 W programme for both pure and binary system of isobutanol and isobutyraldehyde. Dipole moments of the binary mixtures are calculated from the dielectric data using Higasi's method and compared with the ab initio results. Conformational analysis of the formation of hydrogen bond between the isobutanol and isobutyraldehyde is supported from the FT-IR spectra. The average relaxation times are calculated from their respective Cole-Cole plots. © 2007 Elsevier B.V. All rights reserved.