Abstract The endoplasmic reticulum aminopeptidases (ERAP)1 and ERAP2 play a critical role in the production of final epitopes presented by MHC class I molecules. Formation of heterodimers by ERAP1 and ERAP2 has been proposed to facilitate trimming of epitope precursor peptides, but the effects of dimerization on ERAP function remain unknown. In this study, we produced stabilized ERAP1–ERAP2 heterodimers and found that they produced several mature epitopes more efficiently than a mix of the two enzymes unable to dimerize. Physical interaction with ERAP2 changes basic enzymatic parameters of ERAP1 and improves its substrate-binding affinity. Thus, by bringing the two enzymes in proximity and by producing allosteric effects on ERAP1, dimerization of ERAP1/2 creates complexes with superior peptide-trimming efficacy. Such complexes are likely to enhance Ag presentation by cells displaying coordinated expression of the two enzymes.