In computations concerning demixing in multicomponent liquid mixtures, it often is of vital importance to know exactly how the binodal and spinodal curves behave. In the case where a mixture has demixing binaries, these curves sometimes behave differently from what one expects. Therefore, ways of predicting the form of both curves for near-binary compositions are welcome. This work provides formulae that relate the direction of the binodal and spinodal curves to thermodynamic properties of the mixture at binary compositions. Also, the influence of the system temperature on the direction of the curves is briefly discussed. Finally, it is shown that the formula for the binodal curve is in agreement with observations made by others.