A process for preparing N-ethyldiisopropylamine by reacting acetaldehyde with diisopropylamine and hydrogen at elevated temperature and under pressure in the presence of a heterogeneous hydrogenation catalyst, the catalyst being a supported transition metal catalyst comprising Pd and/or Pt as catalytically active metal, wherein the diisopropylamine used has a purity of 58% to 94% by weight and impurities as follows:3% to 20% by weight of water, 3% to 20% by weight of isopropanol, 0% to 2% by weight of others.