Epiquinidine is an alkaloid derived from the bark of the cinchona tree. The most abundant constituents of the Cinchona barks are two pairs of erythro diastereoisomers: quinineand quinidine, which are active antimalarials. Their threo epimers, epiquinine and epiquinidine, are practically inactive. Compared to quinine and quinidine, the 9-epimers had significantly reduced hemozoin inhibition efficiency and did not affect pH-dependent aggregation of ferriprotoporphyrin IX (FPIX) heme. Magnetic susceptibility measurements showed that the 9-epimers perturb FPIX monomer-dimer equilibrium in favor of monomer, and UV-visible (VIS) titrations showed that Epiquinine and Epiquinidine bind monomer with similar affinity relative to quinine and quinidine. However, unique ring proton shifts in the presence of zinc(II) protoporphyrin IX (ZnPIX) indicate that binding of the 9-epimers to monomeric heme is via a distinct geometry.