The asymmetric unit of the title compound, C(9)H(9)N(2) (+)·C(4)H(3)O(4) (-)·0.5C(4)H(4)O(4), consists of one 2-phenyl-imidazolium cation, one hydrogen fumarate anion and half a fumaric acid mol-ecule, which lies on an inversion center. N-H⋯O and O-H⋯O hydrogen bonds connect the cations, anions and fumaric acid mol-ecules into sheets parallel to the (102) plane.