Thin films of porous silicon formed by electrochemically etching silicon wafers changed color when exposed to an ambient atmosphere saturated in various organic solvent vapors. The degree of the color change was related to the refractive indices of the solvents. Analysis of the data using a four‐layer optical model indicated that the film refractive index increased up to 15% when solvent molecules replaced air in the pores. Solvent condensing from the saturated atmosphere filled up to 45% of the total void volume. Thermally oxidizing the films to make them hydrophilic resulted in surfaces which changed color upon exposure to water. ©1996 American Institute of Physics.