We report on room-temperature polarized far-infrared reflectance in pa
rtially ordered GaInP performed with the incidence plane parallel to t
he growth and ordering directions. Measurements in the s-polarization
geometry show only one feature related to the vibrations in the plane
of ordering. On the contrary, those performed using p polarization dis
play a structure appearing in the range of InP-like vibrations as the
order parameter increases, which we relate to a resonance with longitu
dinal-optic mode character due to the anisotropy of the longitudinal p
honons.