The polarized infrared spectrum of naphthalene single crystal has been studied between 800 and 1600 cm(-1). The dependence on temperature of the linewidth of eight Davydov components of internal modes has been studied in the 10-240 K temperature range. The deconvolution of the experimental line shapes allows us to extract a constant inhomogeneous contribution to the line broadening, due probably to crystal defects, over all the temperature range. The bandwidth data are interpreted on the basis of elementary relaxation mechanisms. In particular, at low temperature, the vibrational decay is dominated by three-phonon depopulation processes. Above 80 K, higher order dephasing mechanisms generally contribute to the vibron dynamics. The calculated decay compares well with experimental results. The infrared results are compared with the relaxation data presented for the Raman modes in previous papers.