Thioglycolic Acid and its salts and esters break disufide bonds in the cortex of the hair. After washing, the hair is treated with a mild solution of hydrogen peroxide, which causes new bonds to form, giving hair the rigid structure necessary for permanent waves. - See more at: http://www.cosmeticsinfo.org/ingredient/ammonium-thioglycolate#sthash.mS1tuhPJ.dpuf