The absence of weak detonations in available experience is felt to pose a problem since the well‐known rapid condensation processes encountered in the flow of expanding gases have all the characteristics and present, in fact, examples of weak detonations. As such, they would suggest that the initiation mechanism producing weak detonations will have to be different from that commonly adopted for strong detonations. The physical aspects of rapid condensation processes are discussed and compared with the conditions obtained by Friedrichs for the existence of weak detonations.