Abstract The solid–liquid phase diagrams of binary mixtures of water with a series of tetrabutylammonium α-amino carboxylates, (n-C4H9)4NOOCCH(NH2)R (R=H, CH3, C2H5, n-C3H7, and (CH3)2CH), were examined. It. It was found that all the salts examined could form hydrates with hydration numbers either around 30 or around 39.