The suitability of unhydrogenated soybean oils (55 '%I C 18: 2 and 7 O/n C 18 : 3) for commercial, small-scale, heavy-duty deep-fat frying has been investigated in a laboratory experiment with four unhydrogenated soybean oils, a sunflower oil (68 "10 C 18 : 2) and a hydrogenated soybean fat (0 Yo C 18 : 2) .The fryingprocedure used simulated commercial, heavy-duty deepfat finish-frying of french fries,and comprised intermittent fryingand cooling on five consecutive days. Analysis of the hydrolysis, oxidation and polymerization of the oils and sensory quality of the finish-fried french fries established that the sunflower oil deteriorated more rapidly than the soybean oils. However, the hydrogenated soybean fat was much more stable than the oils. With regard to a limit of 10 "/ o polymeric triglycerides, the frying life of hydrogenated soybean fat was at least twice that of unhydrogenated soybean and sunflower oils. Two of the soybean oils were more stable than the other two. Thus it was concluded that unhydrogenated soybean oils are no less suitable for heavy-duty deep-fat frying than other highly polyunsaturated oils. However, frying conditions, such as temperature, surface/volume ratio and intermittent operation need to be carefully controlled,and the oil needs to be replaced more frequently.