This paper deals with the study of very low incident energy (0–12 eV) electron beam interactions with porous silicon (PS) substrates. It mainly concerns electron stimulated desorption (ESD) of negative H− ions ejected during a resonant bond breaking process. The experiments performed with a specific experimental set-up show that initially the PS surface was repulsive with regards to the low energy electron beam so that a reference voltage v0 = 2.44 V sets the origin of incident electron energy. Thus the resonant desorption process was centred at 7.5 eV with a threshold at 4.6 eV, close to characterisations on the hydrogenated surfaces of silicon crystals. However, the desorption yield was here very low in comparison. It was assumed that a large fraction of the incident electron beam entered the substrate depth enough without triggering significant ion desorption. Progressive shifts in the reference potentials up v0 = 5 V and significant reductions in desorption yield were caused by repeated electron bombardments with energies never exceeding 12 eV. These variations were attributed to changes in the surface composition of the porous silicon. It is assumed that residual fluorine atoms left after PS preparation might migrate towards the porous silicon surface as F− ions to enhance the electron beam repulsion. Following a general discussion, several experiments are suggested in order better to control modification of surface composition responsible for the ageing of the PS substrates.