Differential scanning calorimetry (DSC) is used to study the melting process of three pure fatty acids (palmitic, stearic and oleic) and both their binary and ternary mixtures. The results obtained for palmitic (T = 337 K, DeltaH = 208.2 J g(-1)) and stearic (T = 344 K, DeltaH = 210.8 J g(-1)) acids indicate the existence of a simple process. However. oleic acid presents a reversible solid-solid phase transition (T = 225 K, DeltaH = 3.6 J g(-1)) prior to melting (T = 278 K, DeltaH = 75.5 J g(-1)). The solidification of this acid is a complex process, since a liquid-liquid transformation (T = 272 K, DeltaH = -10.5 J g(-1)) takes place prior to the change of the liquid-solid aggregation state (T = 260 K, DeltaH = -61.0 J g(-1)). The presence of the oleic acid in the binary and ternary mixtures produces melting points at different levels of temperature. The eutectic temperature is determined for the palmitic/stearic mixture (T = 331 K), its non-ideal behavior being demonstrated. The melting heat in the binary mixtures hardly changes, while in the ternary mixtures a lineal decrease is observed when the oleic composition is increased.