The temperature dependence of the quenching cross section of two long-lived metastable states of antiprotonic helium, in collisions with hydrogen and deuterium molecules, has been measured. In the case of the state (n,l) = (37, 34), the cross section decreases with decreasing temperature until it levels off below similar to 30 K, showing a quantum tunneling effect with a small activation barrier. In the case of the state (39,35), on the other hand, for which the lack of an activation barrier was predicted theoretically, the quenching cross section increases slightly with decreasing temperature, indicating the 1/v Wigner threshold law of exothermic reactions involving neutral particles. (c) 2006 Published by Elsevier B.V.