We present experimental results on the instability of thin liquid polydimethylsiloxane films intercalated between a solid substrate and a liquid surrounding medium. The instability is induced by long-range van der Waals forces across the film. From the characteristic wavelength of the instability and from the apparent contact angle of the equilibrium structures, we deduce an effective Hamaker constant (A(132)) about 2-3 orders of magnitude higher than what is expected from calculations based on the individual Hamaker constants. With air as the surrounding medium, we obtain the expected values.