A SAW based pressure sensor comprises a base (2a) having an aperture (3) formed therein and a substrate (4) mounted on the base so as to completely overlie the aperture. The substrate (4) is adhered to the base (2a) so as to form a fluid tight seal around the periphery of the aperture (3). A first SAW resonator (5) is mounted on the substrate (4) wholly within the region overlying the aperture (3), whilst two further SAW resonators (6, 7) are mounted on the substrate wholly within a region which is stiffened by adhesion to the underlying base (2a), such that the two further SAW resonators (6, 7) are completely decoupled from the strain field arising in the substrate due to deflections thereof.