The active components of largely consumed dietary onions (Allium species) in Turkey were extracted with water and ethanol, separately heated in a drying oven (at 50 degrees C, for 14 h) and in a microwave oven (at 90 W, for 14 min), and their total antioxidant capacity was determined with different electron transfer-based assays. Five different onion species/aerial parts, namely yellow, red, white, fresh green leaf, and fresh green root, were measured for total antioxidant capacity with different methods, the hierarchic order in aqueous extracts being: CERAC: yellow-skinned > red-skinned > spring-root > spring-leaf > white-skinned onion; CUPRAC: spring-leaf > red-skinned > spring-root > yellow-skinned > white-skinned onion; Folin-Ciocalteau method: spring-leaf > spring-root > red-skinned > yellow-skinned > white-skinned onion. Using all three methods, white onion, showed the lowest total antioxidant capacity, while with respect to two methods (i.e., CUPRAC and Folin-Ciocalteau), spring onion-leaf showed the highest value. In ethanolic extracts, white onion exhibited the lowest total antioxidant capacity using two methods (CUPRAC and Folin-Ciocalteau). Of the heat-processed onions, the highest CERAC and Folin-Ciocalteau total antioxidant capacity values were obtained for red-skinned onions, while the highest CUPRAC value was for spring onion leaves. All three assays marked white-skinned onion as the lowest total antioxidant capacity content of heat-processed products. The change in total antioxidant capacity caused by both heating processes was not drastic; spring onion leaves essentially maintained its total antioxidant capacity level after 4 min microwave or 4 h drying oven heating. Onion processing by heat treatment did not cause a drastic loss in antioxidant values, favourable for traditional cooking practices.