Abstract: A flame arrester for a pipe line is provided comprising a detonation attenuator mounted within the arrester chamber between the quenching element and the backflash flame inlet. The attenuator is generally cup-shaped, aligned with the inlet, of greater diameter than the inlet but of lesser diameter than the arrester chamber, and is positioned close to the inlet so as to circumscribe it. The major portion of the high pressure central portion of a detonation wave generated by a backflash is received by the cup and reflected back into the pipe. Some of the detonation wave passes around the cup and impinges on the arrester element--however it has been delayed sufficiently to ensure complete quenching of the flame front in the element.