The invention relates to a method and device for measuring gaseous substances, in which the method comprises the stages: ionization of the sample gas in a gas flow, leading of the ionized gas flow through an elongated ion-mobility measuring chamber in the cross-section defined by it, filtering out of ions from the ionized gas flow at a distance from the measuring electrodes, permitting the passage of only the ions travelling from the flow cross-section at the selected point, separation of ions with a different ion mobility, with the aid of a transverse static electric field and at least one measuring-electrode pair arranged along the wall of the measuring chamber.