A wireless and passive surface acoustic wave (SAW) microsensor was developed for simultaneous measurement of and relative humidity (RH). The developed SAW microsensor was composed of SAW reflective delay lines structured by an interdigital transducer (IDT) and several shorted grating reflectors, , and water vapor-sensitive films. A theoretical model was established to predict the response mechanism of the polymer-coated SAW gas sensor. Infusion of into the chamber and humidity changes induced large phase shifts in the reflection peaks. Good linearity and repeatability were observed in the concentration of and RH. The obtained sensitivity was for detection and 7.45°/%RH for humidity sensing. Temperature and humidity effects were also investigated during the sensitivity evaluation process.