Metal-ligand interactions are important in such diverse areas as environmental remediation, drug development, and biochemistry. Potentiometric and spectroscopic techniques, including UV-Visible, infrared, Raman, and NMR spectroscopies, are commonly used to investigate metal-ligand interactions. However, ESI-MS is a valuable technique for accurate determination of complex stoichiometric ratios, and for the study of the gas-phase chemistry of metal complexes with organic ligands. This dissertation presents two broad areas where ESI-MS has been used to study such complexes. In the first area, I investigated the analytical importance of transition metal interactions with phosphorothioate and phosphorodithioate pesticides. These pesticides are important subclasses of organophosphorus pesticides commonly used in agriculture to control pests in fruits and vegetables. Using ESI-MS, I examined how the interactions of three phosphorothioate pesticides (fenitrothion, parathion, and diazinon) and one phosphorodithioate pesticide (malathion) with silver and copper ions affects the mass spectral detection of the resulting complexes. My results show that each pesticide forms silver and copper complexes that significantly improve their detection using ESI-MS. I also found that these metal-pesticide complexes do not undergo the thiono-thiolo rearrangement reaction during collision-induced dissociation, unlike protonated phosphorothioate ions. In the second part of the dissertation I explore the use of ESI-MS for characterization of gas-phase complexes that arise from uranyl(VI), vanadium(V) and iron(III) interactions with 2,6-dihydroxyiminopiperidine (DHIP) and N1, N5-dihydroxypentanediimidamide (DHPD) in aqueous solutions. I also investigated uranyl(VI) interactions with N1, N5-dihydroxyethanediimidamide (DHED). Uranium is an important fuel for nuclear power generation, and there is much interest in the possibility of its extraction from seawater using amidoxime-functionalized sorbents. Vanadium and iron can compete with uranium for binding sites on these sorbents. My results show that DHIP binds uranyl(VI) more effectively that DHPD or DHED, forming ions having uranyl(VI):DHIP stoichiometric ratios of 1:1, 1:2, and 2:3. Vanadium(V) forms 1:1 and 1:2 vanadium(V):DHIP complexes, while iron(III) forms only a 1:2 iron(III):DHIP complex. With DHPD, vanadium(V) forms only a 1:2 vanadium(V):DHPD complex, while iron(III) forms both 1:2 and 1:3 complexes with DHPD. I also observed that gas-phase uranyl(VI)-DHIP complexes are less likely to form in the presence of either vanadium(V) or iron(III).