The thermal phase transitions of 18 pure triacylglycerides (TAGs) and four fish oils were investigated using differential scanning calorimetry (DSC) and modulated DSC (MDSC) in the temperature range from − 150 to 80 °C. The dependence between the heat of fusions and the melting temperatures was determined for pure TAGs, and a regression equation was obtained. A liquid fraction in TAG mixtures and fish oils was determined by the partial integration of melting peaks enhanced by the regression equation. The method was considered to be beneficial for fish oils, because several pure TAGs did not show crystallization. These “unfreezable” TAGs were composited by polyunsaturated fatty acids with three or more double bonds. They showed a glass transition in the temperature range between − 113.9 and − 105.6 °C. The fish oils melted in the range between − 92.6 and 23 °C. The heat of melting was measured to be between 31.5 and 41.8 J g− 1. The oils had a significant share of liquid fraction (between 40.0 and 58.0%) at temperatures down to − 110.0 °C when the glass transition appeared.Copyright © 2014 Elsevier Ltd. All rights reserved.