Our experimental observations show that certain dyes when incorporated in solid polymer electrolytes (SPE) can serve as an excellent candidates for optical humidity sensing. Crystal violet (CV) have been incorporated in poly(vinyl alcohol) (PVA)/H3PO4, which is known to be a SPE and a good proton conductor. Films of PVA/H3PO4 containing CV dye have been studied for use in optical humidity sensing and the interesting results have been discussed here in this paper. We have also tried to give a mechanism for the changes observed in the optical properties of such films by the variation of humidity. © 2001 Published by Elsevier Science B.V.