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 μm thick cavity is generated by a photoelectrochemical etch of single‐crystal (100) p‐Si wafers (3 mA/cm2, 9 C/cm2, irradiated with 57 μW/cm2of 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 μm). 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.