The preparation of the three crystalline potassium salts of fumaric acid, "acid" potassium hydrogen fumarate (2KHC4H2O4.H2C4H2O4), potassium hydrogen fumarate (KHC4H2O4), and dipotassium fumarate dihydrate (K2C4H2O4.2H2O), is discussed, and suitable pH values for the growth of single crystals of each are given. Unit cell, space group, and X-ray diffraction powder identification data for the three salts are presented.