Process for preparing crystalline fumarate and high purity dimethyl

Process for preparing dimethyl fumarate, comprising: reacting: (a) fumaric acid; and (b) methanol; in the presence of sulfuric acid; in a ratio of methanol to fumaric acid of about 5.7 to about 8.6 liters of methanol per kg of fumaric acid; in a reaction mixture to obtain a product mixture comprising a low concentration of dimethyl sulfate, wherein the concentration of dimethyl sulfate is less than 4.0 ppm; where the term "about" is used to indicate the given about 1 to 10% number.