Abstract: A warm air intake system for an internal combustion engine having a change-over valve which selectively connects an air intake passage of the engine to a cold air source or to a warm air source, diaphragm means supplied with intake manifold vacuum by a vacuum passage so as to change-over the change-over valve in accordance with supply or relief of vacuum to its diaphragm chamber, and a snap action thermostat control valve which involves a temperature hysteresis between its on and off operations and which selectively opens a middle portion of the vacuum passage to the atmosphere in accordance with the temperature of the air flowing through the air intake passage.