.beta.-Alaninediacetic acid (Ia) or its alkali metal or ammonium salts (Ib) are prepared by a process in which iminodiacetic acid and acrylic acida) for the preparation of Ia, are reacted with one another in a nonbasic aqueous medium andb) for the preparation of Ib, are reacted with one another in an alkaline aqueous medium or an aqueous medium containing a nitrogen base, or the Ia formed in the nonbasic medium is converted with an alkali metal base, ammonia or an amine into Ib.