The paper describes the detailed transition mechanism from steady to unsteady flow in a multilouvered fin geometry. The initial instability appears in the wake of the exit louver at a Reynolds number of 400 with a characteristic non-dimensional frequency (based on inflow velocity and louver pitch) of 0.84. Between a Reynolds number of 700 and 800, power spectra in the interior of the array indicate an increase in energy in the vicinity of the first harmonic of the initial exit wake instability. By a Reynolds number of 900, free shear layer or Kelvin-Helmholtz type instabilities develop on the leading edge shear layers of louvers near the exit. These instabilities have a characteristic non-dimensional frequency of 1.7. As the Reynolds number increases further, instabilities move upstream into the array. By a Reynolds number of 1300, most of the louvers exhibit unsteadiness, except for the entrance louver and the first two louvers following it. By this Reynolds number. the flow in the downstream half of the array exhibits a chaotic behavior. (C) 2000 Elsevier Science Ltd. All rights reserved.