An impact of inhomogeneities on photoluminescence (PL) of porous silicon (PS) is analysed using numerical simulation and supported experiment under gas adsorption. Depending on the excitation wavelength and the condition of measurement (steady-state or transient mode), the gas adsorption can result in the quenching or increase of FL. The emission efficiency of the as-prepared porous layers is shown to decrease at the adsorption of acetone molecules for each excitation wavelength in the 405-546-nm range. However, if the 546-nm excitation causes small change of the photoluminescence spectrum during exposition, the 405-nm excitation quenches the PL in the ambient air and increases emission efficiency in acetone vapours. Effects are discussed with the model approach of different recombination properties, and the contribution to the PL incoming of the upper photooxidised and bottom nanocrystalline layers. We describe the method of the PL spectra fitting in order to determine the gradient profile of porous layers having depth irregularity of PL.