Most real solutions exhibit deviations from Raoult's law. A positive deviation is characterized by and and indicates that the attractive interactions between like molecules is greater than that between and molecules. A negative deviation has and , implying stronger mutual interactions between unlike molecules. The curves shown in the graphic are qualitative approximations to the actual dependence of vapor pressures on composition. The blue and red curves represent the partial pressures of and , respectively, while the black curve shows the total vapor pressure. The dashed lines refer to the hypothetical ideal behavior of the corresponding vapor pressures.

Even for nonideal solutions, Raoult's law is asymptotically approached for or . Dilute solutions, on the other hand, are approximated by Henry's law: the linear relations for and for , which can be displayed using the "show Henry's law" checkbox.