The effect of chemical surface treatment on the sensitivity, specificity, and stability of mesoporous Si thin film vapor sensors is studied. The vapor sensors operate by measurement of Fabry-Perot interference from the porous Si layer, probed using a diode laser operating at a wavelength of 687 nm. Four chemically distinct surface types are each probed with three different analyte vapors: ethanol, methyl ethyl ketone, and n-hexane, all in a carrier gas of pure nitrogen. The four different surface types include the H-terminated, freshly etched material (Si-H), ozone-treated material (Si-ozone), electrochemically methylated material (Si-CH3), and thermally oxidized samples (Si-O-Si). Surface modification has a pronounced effect on the specificity and stability. It is found that the Si-H material is more sensitive to the hydrophobic analyte relative to either the Si-O-Si or Si-ozone samples. Similarly, the Si-CH3 material is more sensitive to the more hydrophobic analyte, although it is found to be much more stable than the Si-H material.