Real time monitoring of contamination on antireflection (AR) silica coatings in high peak power laser systems (HPLs) is critically needed in order to avoid reductions of transmission and laser damage to optical surfaces. Herein we proposed to apply a surface acoustic wave (SAW) sensor to real-time monitor trace amounts of airborne molecular contaminants (AMCs) adsorbed on the AR silica coatings. The silica coating is found to be susceptible to AMCs because of its mesoporous structure, huge surface area and polar nature. The adsorbed AMCs caused the increased mass on the silica coating of the SAW sensor, which resulted in a significant increase of its frequency shift. The fabricated sensor showed a high sensitivity of ~-490 mm2ng-1Hz and an excellent linearity vs. the areal density of adsorbed AMCs since the frequency shift of the sensor is linearly related to the change of mass of the silica coating.