Differential scanning calorimetry (DSC) was used to study the freezing of jet fuel and the effect of cold flow improving additives. We find that the cooling (freezing) exotherm is a more useful diagnostic tool for this purpose than the heating (melting) endotherm. Jet fuels (Jet A, JP-8, and Jet A-1) display a strong exotherm upon cooling between -45 and -60 degreesC. By the study of mixtures composed of classes of jet fuel components (normal paraffins, isoparaffins, and aromatics), we find that the cooling exotherm is primarily due to the liquid-solid phase transition of the normal paraffins. Cold flow improving additives (e.g., pour point depressants) show only small effects on the DSC exotherm despite larger effects observed in cold flow devices. This indicates that these additives work primarily by affecting the habit of the n-alkane crystals. This change in crystal habit is supported by low-temperature microscopy studies.