Uniform layers of porous Si have been produced that show fine structure in their photoluminescence (PL) spectra characteristic of longitudinal optical cavity modes. This mode spacing can be modified in a predictable way by immersing the porous Si layer in heptane, which changes the average refractive index of the porous Si layer. The 5 mu m thick cavity is generated by a photoelectrochemical etch of single-crystal (100) p-Si wafers (3 mA/cm(2), 9 C/cm(2), irradiated with 57 mu W/cm(2) of 500 nm light for the duration of the etch). Electron micrographs show that the layer is homogeneous on a submicron scale and that the Si/porous Si and porous Si/air transitions are abrupt (<0.5 mu m). On thinner porous Si films, optical interference leads to a significant distortion of the emission spectrum. The relevance of interference-induced spectral changes to measurements of the intrinsic emission spectrum of porous Si is discussed.