IR absorption measurements have been made using a high resolution Fourier-transform spectrometer on crystals of InP in the 400-4000 cm-1 region at temperature between 11-300 K. Very weak new absorption bands are observed, their absorption coefficients are ～10-1 cm-1, were observed at 996cm-1, 965cm-1, 932cm-1, 838cm-1, 776 cm-1, 742cm-1, 718cm-1,590cm-1, 558cm-1 and 538 cm-1. The frequencies and temperature dependence of these weak bands indicate that they arise from three-phonon processes and appropriate assignments are given. Using IR absorption technique, we observed for the first time the boron contamination in LEC(B2O3)-CZ InP crystals. The content of baron contamination has been proved to be ～10-16cm-1.