We present a principle of discriminating dialdehydes from monoaldehydes by using optical responses of liquid crystals (LCs) triggered by a surface reaction between aldehydes and surface amine groups. When a solid surface is decorated with a mixture of long hydrocarbons and primary amines, a surface reaction between dialdehydes and the surface amines generates an aldehyde-terminated surface (which can be tested by using triglycine-copper complex as an aldehyde marker). This type of surface is able to trigger an ordering transition of 4-cyano-4'-pentylbiphenyl (5CB) supported on the surface. In contrast, the surface reaction between monoaldehydes and the surface amines leads to a hydrocarbon-terminated surface which does not influence the orientations of 5CB on the surface. Because LCs are birefringent materials, the changes of orientations can cause distinct optical textures visible to the naked eye. The methodology reported herein provides a simple optical method of detecting and distinguishing dialdehydes from monoaldehydes with high sensitivity (~ 10 μM for glutaraldehyde and phthaldialdehyde) and specificity. Based on this principle, we develop a LC-based optical sensor for detecting glutaraldehyde vapor by using real-time orientational responses of 5CB to glutaraldehyde. Because the performance of the LC-based sensor critically depends on the diffusion of glutaraldehyde in 5CB, we measured the solubility and diffusivity of glutaraldehyde in 5CB. Our experimental results show that an optimal LC sensor with a 20 μm-thick film of 5CB can respond to 207 ppmv glutaraldehyde within 20 s and show a full response in 4 min. It also shows good specificity. Only a very weak response is observed when this sensor is exposed to acetic acid, and no response is recorded for ethanol or methanol.