The failure of a number of fins of a large two-stage reciprocating compressor intercooler was studied. It appeared that leakage flow across the ends of the fins excited the vibrations and eventually led to fatigue failure. A series of simple laboratory tests were conducted to confirm this excitation mechanism. It is seen that the fins may be excited to resonance by coupling with the shear layer oscillations in several ways. The heat exchanger problem was cured by using sealing strips to prevent the leakage flow.