This study focuses on the use of waste cooking oil (WCO) as the main feedstock for hydrotreatment to evaluate the effect of temperature on the product hydrocarbon composition. A qualitative analysis was initially performed using a GC×GC–TOFMS indicating the presence of mainly paraffins of the C15–C18 range. A quantitative analysis was also performed via a GC–FID, which gave both n-paraffins and iso-paraffins in the range of C8–C29. The results indicate that hydrotreating temperature favors isomerization reactions as the amount of n-paraffins decreases while the amount of iso-paraffins increases. For all experiments the same commercial hydrotreating catalyst was utilized, while the remaining operating parameters were constant (pressure=1200psig, LHSV=1.0h −1 , H 2 /oil ratio=4000scfb, liquid feed=0.33ml/min, and gas feed=0.4scfh).