Properties of propagation modes for a layered ZnO/XZ LiNbO 3 surface acoustic wave sensor have been investigated both theoretically and experimentally. By calculating the effective permittivity func-tion, it was shown that after the ZnO deposition two distinct propagation modes have been gener-ated: a Rayleigh and a shear horizontal. A layer of InO x was deposited to provide a conductometric sensing mechanism to NO 2 . It was found that the sensor is at least one order of magnitude more sensitive when operating in Rayleigh mode than in the shear horizontal mode. This sensitivity was found to be in good agreement with calculations for low concentrations of NO 2 , however discrepan-cies were observed for higher concentrations.