Chapter One is an introduction to the characteristics of gold thiolate-protected nanoparticles, including synthesis, optical and electrochemical properties, and structure. A brief history and study of nanoparticle detection using mass spectrometry is also discussed. Chapter Two discusses the Electrospray ionization mass spectrometry (ESI-MS) of Au25L18, where L is a thiolates ligand. The study of past experiments with Au25(SC2H4C6H5)18 with ESI-MS is reviewed, along with recent studies of Au25 products modified with various ligands and charge agent tags, including carboxylic acid-functionalized thiols and inorganic salts. Chapter Three describes the synthesis and characterization of Au24Pd(SC2H4C6H5)18 using electrochemistry and mass spectrometry. Despite a change of only one atom versus Au25(SC2H4C6H5)18, the change in properties is drastic. Chapter Four explores the reactivity of Au25(SC2H4C6H5)18 with Ag+, Cu2+, and Pb2+, which results in changes in absorbance and fluorescence spectra and voltammetry. Mass spectrometry of Ag+ titration products indicates the formation of Au24Ag(SC2H4C6H5)18, Au23Ag2(SC2H4C6H5)18, and Au22Ag3(SC2H4C6H5)18. The proposed mechanism for reaction is a reversible redox model where the nanoparticle reduces the metal ion forming Au25M(SC2H4C6H5)18 adducts which form bimetals in the oxidative environment of mass spectrometer's ionization source. Chapter Five describes the collision-induced dissociation tandem mass spectrometry (CID MS/MS) of [NaxAu25(SC2H4Ph)18-y(S(C2H4O)5CH3)y]x-1, which reveals that the primary dissociation pathway involves the outer protecting structures, or semi-rings Au2L3, of the nanoparticle core. Fragments produced under CID conditions are also seen under non-CID conditions, proving that even soft ionization results in fragmentation. Chapter Six presents MALDI mass spectra and analytical data which identify and characterize [Au25(SC2H4C6H5)18]1- chelated by CH3C6H3(SH)2. Optical and electrochemical properties indicate obvious changes upon initial displacement of -SC2H4C6H5 with the dithiol, which MALDI shows to be due to some chelation of the semi-rings. Later in the exchange process, this chelation cannot compete with the dominance of single thiol coordination. Chapter Seven describes the Electrospray ionization triple quadrupole mass spectrometry (ESI-QQQ-MS) of approximately 1.6 nm diameter thiolate-protected gold nanoparticles, revealing a mixture of nanoparticles with formulae Au144L60 and Au146L59. The nanoparticles are cationized by exchanging multiple [-SC11H22N(CH2CH3)3+] ligands into the original [-S(CH2)5CH3] ligand shell.