The mitochondrial inner membrane peptidase Imp is required for proteolytic processing of the mitochondrially encoded protein Cox2, the nucleus-encoded Cyt b2, Mcr1, and Cyt c1, and possibly other proteins, during their transport across the mitochondrial membranes. The peptidase contains two catalytic subunits, Imp1 and Imp2. The small protein Soml was previously shown to affect the function of Imp1, but the precise role of Soml remained unknown. Using mutants deleted for IMP1, IMP2 and SOM1, we show here that the Som1 protein is absent in the imp1delta mutant, whereas the level of the Imp1 subunit of the peptidase is only slightly reduced in the soml null mutant. The Soml protein is not essential for proteolytic processing of Cyt b2, while the two other known Imp1 substrates, Cox2 and Mcr1, are not processed in the absence of Som1. Proteolytic processing of Cyt c1 by the Imp2 subunit, and of Ccp by an as yet unidentified peptidase, is not impaired in the som1 deletion mutant. By crosslinking and co-immunoprecipitation assays we demonstrate that the Imp1 and Som1 proteins physically interact. We conclude from our results that stabilisation of Som1 and correct Imp1 function is mediated by a direct interaction between the Imp1 and Som1 proteins, suggesting that Som1 represents a third subunit of the Imp peptidase complex.