RNA polymerases (Pol) II and III synthesize eukaryotic mRNAs and tRNAs, respectively. The crystal structure of the 12 subunit Pol II is known, but only limited structural information is available for the 17 subunit Pol III. Using mass spectrometry (MS), we correlated masses of Pol II complexes with the Pol II structure. Analysis of Pol III showed that the complete enzyme contains a single copy of each subunit and revealed a 15 subunit form lacking the Pol III-specific subcomplex C53/37. DMSO treatment dissociated the C17/25 heterodimer of Pol III, confirming a peripheral location as its counterpart in Pol II. Tandem MS revealed the Pol III-specific subunits C82 and C34 dissociating as a heterodimer. C11 was retained, arguing against a stable trimeric subcomplex, C53/37/11. These data suggest that Pol III consists of a 10 subunit Pol II-like core; the peripheral heterodimers C17/25, C53/37, and C82/34; and subunit C31, which bridges between C82/34, C17/25, and the core.