The invention relates to a process for preparing low-by-product, light-color methylglycine-N,N-diacetic acid tri(alkali metal) salt by alkaline hydrolysis of methylglycinediacetonitrile (MGDN), comprising the steps in the sequence (a) to (f): (a) mixing of MGDN with aqueous alkali at a temperature of .ltoreq.30.degree. C.; (b) allowing the aqueous alkaline MGDN suspension to react at a temperature in the range from 10 to 30.degree. C. over a period of from 0.1 to 10 h to form a solution; (c) allowing the solution from step (b) to react at a temperature in the range from 30 to 40.degree. C. over a period of from 0.1 to 10 h; (d) optionally allowing the solution from step (c) to react at a temperature in the range from 50 to 80.degree. C. over a period of from 0.5 to 2 h; (e) optionally allowing the solution from step (c) or (d) to react at a temperature in the range from 110 to 200.degree. C. over a period of from 5 to 60 min; (f) hydrolysis and removal of ammonia of the solution obtained in step (c), (d) or (e) by stripping at a temperature of from 90 to 105.degree. C.