The Fourier Transform Infrared spectra of crystallized phenoxyacetic acid and p-chloro derivative have been recorded in the region 4000-400 cm -1 . The geometry was calculated by Hartree-Fock (HF), Density Functional Theory (B3LYP) and Moller Plesset(MP2) Perturbation theory invoking 6-311++g(d,p) basis set. The harmonic vibrational frequency and, infrared intensities have been investigated with the help of B3LYP methods. The scaled theoretical wavenumbers showed very good agreement with the experimental values. The thermodynamic functions of the title compounds were also performed at B3LYP/6-311++g(d,p) level of theory. A detailed interpretation of the infrared spectra of Phenoxyacetic acid and p-chloro derivative of Phenoxy acetic acid have been reported.