A method for the synthesis of 5-aryl-1,3,3,-trimethyl-2-methylene-indoline derivatives of Formula (I) or its salts of Formula (Ia) ##STR00001## wherein 1 mole of a 5-aryl-2,3,3-trimethyl-3H-indole derivative of Formula (VII) is reacted for 1 to 44 hours at a temperature of 20.degree. to 180.degree. C. in an apolar, aprotic or polar, protic or polar aprotic solvent with 1 to 50 moles of a compound of Formula R1-A; new compounds of Formulas (I)/(Ia) and (V), obtainable by this method ##STR00002## as well as an agent containing at least one compound of Formula (I)/(Ia) and a carbonyl/imine compound for dyeing keratinic fibers and a method for temporarily dyeing keratin fibers, for which the keratin fiber is dyed with the aforementioned agent and the dyeing, so obtained, is removed again at any later time by a sulfite preparation.