We study the onset of Marangoni instability of the quiescent equilibrium in a binary liquid layer with a nondeformable interface in the presence of the Soret effect. Linear stability analysis shows that both monotonic and oscillatory long-wavelength instabilities are possible depending on the value of the Soret number χ. Sets of long-wavelength nonlinear evolution equations are derived for both types of instability. Bifurcation analyses reveal that in the regime of monotonic instability square patterns bifurcate supercritically and they are preferred in competition with roll patterns. Hexagonal patterns bifurcate transcritically and the condition for the emergence of steady stable hexagonal patterns is derived. In the case of oscillatory instability, traveling and standing waves are found to bifurcate supercritically in the narrow range of the Soret parameter and traveling waves are found to become the selected type of flow.