Abstract: Disclosed herein are a process for producing mixed crystals of disodium 5′-guanylate and disodium 5′-inosinate which comprises precipitating mixed crystals of disodium 5′-guanylate and disodium 5′-inosinate (I+G mixed crystals) by adding an aqueous mixed solution of disodium 5′-guanylate and disodium 5′-inosinate and a hydrophilic organic solvent at the same time into a crystallization vessel in such manner that the ratio of the hydrophilic organic solvent to the liquid phase in the crystallization vessel is maintained in a range of 30 to 70 vol %, as well as such process for producing I+G mixed crystals wherein said producing of I+G mixed crystals is carried out by seeding crystallization wherein crystals of 5′-IMP2Na or/and I+G mixed crystals are used as seed crystals.