Cold flow properties have historically been important for diesel and jet fuels. Reflecting the importance of cold flow properties, several standards have been developed to characterize pour point, cloud point, and filterability. An emphasis on characterizing fuels based on standard testing methods has led to large amounts of data that describe how fuels perform but very little published data that describe what is happening at the molecular level and to the composition of fuels.Motivated by a desire to have an improved understanding of the cold flow behavior of Fischer-Tropsch fuels, an experimental method was developed to provide easy acquisition of data on the changing compositions of liquid and solid phases as Fischer-Tropsch and diesel fuels traverse cloud point, pour point, and additive-enhanced pour point temperatures. These data provide an insight into the fundamental driving force leading to cold flow behavior manifesting itself as cloud points, pour points, and filterability of diesel fuels by Low Temperature Flow Test (LTFT). Two freezing point depression theory models were compared to the data to identify the relations between composition and cold flow behavior.An improved understanding of cold flow properties is most important when using Fischer-Tropsch liquids (FTL) as fuels. FTL contain high fractions of C20+ paraffins as well as high fractions of C9- paraffins and alkenes to increase the fractions of C20+ soluble in the liquid phase. This paper evaluates the cold flow behavior and ignition temperature limits for this important class of fuels.To avoid storing fuel with vapors between the upper and lower ignition temperature limits, inclusion of naphtha down to C6 appears to be a prudent safety precaution. These fuels would require handling precautions similar to those used for gasoline. The purposes of this paper are to (1) provide an improved understanding of cold flow behavior and (2) assess the broad-cut product approach of using Fischer-Tropsch fuels in diesel engines. The broad-cut approach retains naphtha and waxy fractions in the fuel as an alternative to the hydro-isomerization approach that changes the molecular structure of Fischer-Tropsch reaction products.