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The viscoelastic properties of thin polymer layers coated on a thickness shear mode (TSM) resonator is investigated by analysis of the impedance spectra of the resonator. The response of resonators coated with polyethleneoxide (PEO) and polydimethylsiloxane (PDMS) to organic vapours is described. The PEO coated sensor shows the conventional Sauerbrey behaviour of a negative frequency shift with adsorbed mass, but the PDMS coated sensors show a strong positive frequency shift. The complex shear modulus for these layers is calculated and it is found that the PDMS is in the rubbery state. The in-diffusion of organic vapours leads to a further softening of the layer which produces the positive frequency shift.