We report the first collision-induced dissociation tandem mass spectrometry (CID MS/MS) of a thiolate-protected Au nanoparticle that has a crystallographically determined structure. CID spectra assert that dissociation pathways for the mixed monolayer NaxAu25(SC2H4Ph)(18-y)S(C2H4O)(5)CH3)(y) centrally involve the semi-ring Au2L3 coordination (L = some combination of the two thiolate ligands) that constitutes the nanoparticle's protecting structure. The data additionally confirm charge state assignments in the mass spectra. Prominent among the fragments is [Na2AuL2](1+), one precursor of which is identified as another nanoparticle fragment in the higher m/z region. Another detected fragment, [Na2Au2L3](1+), represents a mass loss equivalent to an entire semi-ring, whereas others suggest involvement (fragmentation/rearrangement) of multiple semi-rings, e.g., [NaAu3L3](1+) and [NaAu4L4](1+). The detailed dissociation/rearrangement mechanisms of these species are not established, but they are observed in other mass spectrometry experiments, including those under non-CID conditions, namely, electrospray ionization mass spectrometry (ESI-MS) with both time-of-flight (TOF) and FT-ICR analyzers. The latter, previously unreported results show that even soft ionization sources can result in Au nanoparticle fragmentation, including that yielding Au4L4 in ESI-TOF of a much larger thiolate-protected Au-144 nanoparticle under non-CID conditions.