The compound Ba3Ca1.18Nb1.82O9-delta belongs to the class of high-temperature proton conductors (HTPCs) which absorb water vapor and become good proton conductors. Oxygen from the water molecule fills vacancies present in the dry state of the above HTPC and the two protons are incorporated in the electron cloud of two O-atoms. A positive linear lattice parameter change was demonstrated in this work when the dry compound was exposed to water vapor. The slope of (Delta a/a)/n(H2O) was determined to be (3.76+/-0.10)x10(-2). Defect structure considerations also favor an increase in the lattice parameter when O-vacancies are filled. The experimentally observed increase is compared with that observed in metals when they are charged with hydrogen.