Positive deviation: A-B interactions weaker than average of A-A and B-B. Molecules escape more easily. $P_{total}$ > Raoult prediction. delta_H_{mix} > 0 (endothermic), delta_V_{mix} > 0 (volume expansion). Examples: ethanol + water (H-bonds disrupted), acetone + CS2, benzene + acetone. Forms minimum boiling azeotrope. Negative deviation: A-B interactions stronger than A-A and B-B. Molecules held more tightly. $P_{total}$ < Raoult prediction. delta_H_{mix} < 0 (exothermic), delta_V_{mix} < 0 (volume contraction). Examples: acetone + chloroform (H-bond formation between C=O and C-H), HCl + water, HNO3 + water. Forms maximum boiling azeotrope.