Abstract: An etch solution for aluminum and the alloys thereof, after being used in caustic etching is regenerated, without using a large amount of heat energy for the concentration of the solution and without causing the accumulation of impurities in the etch solution, by(a) diluting at least a portion of an etch solution, after being used in caustic etching of aluminum or the alloys thereof, containing 100 through 240 g/l of free sodium hydroxide and 30 through 200 g/l of dissolved aluminum which is in an unsaturated state, as aluminum hydroxide, or in a similar condition by adding water in an amount of 0.5 through 2.