Deionized or DI water is free from dissolved minerals such as mineral salts, including iron, calcium, and sulfate. It is also termed ASTM D 1193 Type IV water and differs from distilled water in terms of purification methods and composition. It is safe for human use but is unfit for drinking purposes because of the absence of essential nutrients required by the human body. Our premium quality Deionized Water is highly recommended for educational and research labs to obtain consistent and repetitive results. Due to the presence of impurities, it is not recommended for use in foods, drugs, and medicines. Assuming the source water was tap or spring water, distilled water is pure enough for nearly all lab applications. It is used for: a solvent to prepare a solution analytical blank calibration standard cleaning glassware equipment sterilization making high purity water The purity of deionized water depends on the source water. Deionized water is used when a soft solvent is needed. It is used for:  cooling applications where it's important to avoid depositing minerals microbiology autoclaves many chemistry experiments involving ionic compounds washing glassware, especially the final rinse solvent preparation analytical blanks calibration standards in batteries As you can see, in some situations either distilled or deionized water is fine to use. Because it is corrosive, deionized water is not used in situations involving long-term contact with metals.