Porous GaP, InP and GaAs structures fabricated by MeV ion-implementation-assisted electrochemical etching were investigated by Raman and Fourier transform infrared spectroscopy. Fröhlich modes in the frequency gap between the transverse optical and longitudinal optical frequencies were observed and their longitudinal-transverse splitting was established. The frequency-dependent optical properties in the infrared region were calculated using a dielectric function derived on the basis of an appropriate two-dimensional effective-medium theory. The theoretical reflectance spectra are found to be in good agreement with the experimental ones and the predicted coupled Fröhlich-plasmon modes for conducting samples were observed experimentally. The wavelength used in Raman measurements did not fulfil the requirements of effective-medium theory, but the resulting spectra could be explained at least qualitatively by taking into account the diffuse scattering.