Poly(A)-specific ribonuclease (PARN) is a deadenylase that degrades the poly(A) tail of eukaryotic mRNA. PARN also interacts with the 5’-cap structure of the mRNA. The binding of the cap structure enhances the deadenylation rate. PARN has previously been described as a dimer. We have studied PARN with size exclusion chromatography to investigate the oligomeric composition and revealed oligomeric compositions of PARN that are larger than dimeric PARN. Deadenylation assays have been used to measure the cap stimulated activity of PARN. The deadenylation assays showed that the cap stimulated activity of PARN correlated with the abundance of oligomers corresponding in size to tetrameric PARN. We present a model for tetrameric PARN and propose a mechanistic model for how the cap stimulates PARN mediated deadenylation.