Room temperature infrared (IR) optical responses of wurtzite indium gallium nitride (InxGa1-xN) in the composition range of 0.174 <= x <= 0.883 were investigated by the polarized IR reflectance spectroscopy. Analyses of the amplitudes of oscillation fringes in the non-reststrahlen region revealed that the high frequency dielectric constants of the samples were unusually smaller than the values predicted from the Clausius-Mossotti relation. This odd behavior was attributed to the porous surface morphology of the InxGa1-xN samples. The E-1 optical phonon modes of the InxGa1-xN were deduced from the composition dependent reststrahlen features. The obtained values were compared to those calculated through the modified random element iso-displacement (MREI) model. The deviation between the measured data and the MREI model prediction were explained in detail from the aspects of strain, thermal expansion and anharmonic phonon-coupling. Finally, it was found that the large discrepancy of the E-1(LO) mode is mainly attributed to the effects of the longitudinal phonon-plasmon coupling. (C) 2016 Elsevier B.V. All rights reserved.