In situ, real-time monitoring of a dehydration of poly(vinyl alcohol) (PVA) film is researched. The technique based on the incorporation plasmonic nanocages (NCs) between the two identical layers of PVA, (PVA)/NCs/(PVA) film, and its sensitivity to the variation of the refractive index of the surrounding PVA film via desorption water. The dehydration time increases from 180 (s) to 1800 (s) as the content of the PVA in the films increases twice, from (PVA)/NCs/(PVA) to (PVA)2/NCs/(PVA)2. Such effect could be explained by different rate of the molecules desorption from the PVA based films. Specifically, the dehydration rate is of 0.22 (vol% per s) and 0.026 (vol% per s) for (PVA)/NCs/(PVA) and (PVA)2/NCs/(PVA)2 films, respectively. The dehydration rate constant reduces from -50×10-4 (s-1) to -4.3 ×10-4 (s-1) as the content of PVA increases from (PVA)/NCs/(PVA) to (PVA)2/NCs/(PVA)2 films, respectively.