Two polymorphs of the 1:1 fumarate salt of 1,4-diazabicyclo[3.2.2]nonane-4-carboxylic acid 4-bromophenyl ester, developed for the treatment of cognitive symptoms of schizophrenia and Alzheimer disease, have been characterized. The 2 crystal structures have been solved, and their phase relationships have been established. The space group of form I is P21/c with a unit-cell volume of 1811.6 (5) Å3 with Z = 4. The crystals of form I were 2-component nonmerohedral twins. The space group of form II is P21/n with a unit-cell volume of 1818.6 (3) Å3 with Z = 4. Relative stabilities have been inferred from experimental and topological P-T diagrams exhibiting an overall enantiotropic relationship between forms I and II although the solid–solid transition has never been observed. The slope of the I-II equilibrium in the P-T diagram is negative, form II is the stable phase below the solid–solid transition temperature of 371 K, and form I exhibits a stable melting equilibrium. The I-II transition temperature has been obtained from the intersection of the sublimation curves of the 2 solid forms.