Infrared (IR) reflection spectra of GaN1-xPx alloys, grown by light-radiation heating, low-pressure metalorganic chemical vapor deposition, have been investigated with phosphorus composition x =0-0.15. A multi-oscillator model has been used to fit the IR reflection spectra. In addition to the demonstration of a GaN-like transverse-optical (TO) mode, a GaP-like TO mode, located at about 355 cm(-1), has been identified for x > 0, showing a typical two-mode behavior. With the increment of x the frequency of the GaN-like TO phonon decreases and the GaP-like TO phonon keeps its frequency almost unchanged within the measured composition range. A simple randomelement-isodisplacement model has been employed to interpret the frequency change of both GaN-like and GaP-like phonons with the phosphorous composition x. The damping of the GaN-like TO mode increases significantly with x and is explained by the disorder effect resulting from the introduction of foreign P atoms. (C) 2004 American Vacuum Society.