An internal combustion engine is equipped with an air line for admitting supplementary air to the induction manifold or the exhaust manifold in order to compensate for an intentional air deficiency, so as to make the mixture stoichiometric. The air line is throttled by a pneumatically activated throttle valve which is subject to at least two control pressures, one of these being the atmosphere or a constant pressure and the other being, for example, the induction tube vacuum. The termini of the two lines which admit these control pressures are so disposed in the valve as to be jointly openable and closable, in opposite phase, by an electromagnetically controlled valve-closing leaf-spring. The electromagnet is energized by a current of variable frequency, the frequency being dependent on the exhaust gas composition as monitored by an oxygen sensor.