Patterns of parallel strips, consisting of alternating polystyrene (PS) and poly(vinyl pyrrolidone) (PVP) regions, were observed in thin films spin cast from a PS/PVP/chloroform solution on unpatterned substrates. The formation of anisotropic patterns, manifested not only in thickness variation but also in composition variation, was found to be driven by Marangoni instability, with the PS and PVP streams flowing toward the preferred regions as the phase separation induced by solvent evaporation proceeded. The initial viscosity of the polymer solution and the thickness of the spin-cast films were lumped into one single parameter to study the phase morphology development at various initial polymer solution concentrations. Interestingly, the ratio of the square of the film thickness to the viscosity, a parameter loosely related to the Marangoni number, was found to reach a maximum value at the concentration where the strip patterns were most well-developed.