An ion mobility spectrometer for analysing a gas is described which comprises a concentric arrangement of an inner ionization region 110, an annular reaction region 112 which surrounds the ionization region 110 and through which a gas to be analyzed is caused to flow continuously in the axial direction of the spectrometer, and an annular drift region 116 surrounding the reaction region through which ionised molecules of the gas to be analyzed flow in a radial direction towards a cylindrical detector 124 forming the outer wall of the drift region 116.