We study theoretically the mechanisms of dewetting of liquid A deposited (by solvent evaporation) on an immiscible, not wettable, liquid B (S = gamma(B) - (gamma(A) + gamma(AB)) < 0, where gamma(ij) are the interfacial tensions). The A film is unstable below a critical thickness e(v) (approximately mm) controlled by gravity effects. Macroscopic films (e > mm) are metastable and evolve by nucleation and growth of holes (radius R(t)) surrounded by a rim, which moves at velocity V = dR/dt. Depending on the viscosities (eta(A), eta(B)), the densities (rho(A), rho(B)) of the two liquids, and the A film thickness, e, we expect four regimes : (i) inertial (V = (Absolute value of S/rho(A)e)1/2; (ii) viscous with "A" friction dominant (V = Absolute value of S theta(E)/eta(A)); (iii) viscous with "B" friction dominant (V = Absolute value S/eta(B)); (iv) viscous inertial (V = Absolute value of S 2/3/(eta(B)rho(B)l)1/3); where l is the width of the rim and theta(E) the equilibrium contact angle (assumed <1). Microscopic films are unstable and dewet by amplification of longitudinal capillary waves. We study this "spinodal decomposition" regime driven by long range van der Waals forces, when A is more polarizable than B. For low viscosity substrates we expect viscoinertial modes. The most unstable mode has wave vector q(m) = a/e2, where a is a molecular length. We expect that a cellular pattern of A droplets is formed with cell size q(m)-1.