Refined soybean oils were degraded at 190°C for 60 h to a total polar material of 61.54%. The effect of fresh to highly degraded oil on the physical and thermal properties of the oil as well as on the final quality of tortilla chips was determined. Surface tension decreased significantly with oil degradation (P < 0.05). Colour readings also changed significantly with oil degradation (P < 0.05) and were more pronounced after 30 h of frying. Foam and off-flavour developed between 20 and 30 h of degradation suggesting that the oil should have been discarded when the total polar material reached 27%. The convective heat transfer coefficient changed more rapidly after 30 h of frying. The values of the convective heat transfer coefficient correlated highly with viscosity (-0.98). The oils, whether fresh or degraded, behaved as Newtonian fluids. Viscosity was significantly affected by oil degradation time and temperature (P < 0.05). The effect of temperature on the viscosity of the oils was described by an Arrhenius equation. Viscosity changed more rapidly with temperature as the degradation time increased. Tortilla chips were fried in fresh and degraded soybean oils. Total oil content and fracturability of tortilla chips were not affected significantly by oil degradation time (P < 0.05). However, the oil content adhering to the surface of the tortilla chips was significantly higher (P < 0.05) in the tortilla chips fried in degraded oil than in fresh oil.