The objective of the paper was to follow through changes in the antioxidant properties of ‘Red Kidney’ beans during their hydro-thermal processing: boiling, autoclaving, and extrusion, connected with converting bean seeds into instant flour. The content of total polyphenols and the antioxidant activity were determined in raw seeds and after the successive stages of hydro thermal processing with the use of ABTS^•+ and DPPH^• methods. Furthermore, a sensory analysis of the instant flour types produced was carried out. It was found that all the processing procedures of converting seeds into flour caused the content of polyphenols to decrease and the antioxidant activity in the final products to drop. The smallest losses were observed during the process of extrusion and amounted to approx. 20–25% compared to the raw material. The antioxidant properties of instant flour types produced using boiling or autoclaving were comparable. The flour type produced using autoclaving turned out to show the most attractive sensory attributes among all other flour products.