Porous layers and free-standing membranes were fabricated by anodic etching of n-GaP substrates in a sulphuric acid solution. Micro-Raman analysis of the interaction between the longitudinal optical phonons and plasmons in porous membranes allowed us to obtain specific information about the electro-optical properties of microstructured GaP. In particular, apart from the carrier exhausted areas surrounding the pores, the existence of conductive regions was demonstrated. A comparative analysis of the secondary electron and panchromatic cathodoluminescence (CL) images evidenced an increase in the emission efficiency caused by porosity. Data concerning the spectral distribution of CL in bulk and porous samples are presented.