We demonstrate the synthesis of discrete, composition-tunable gold-cobalt nanoparticle alloys (% Co = 0-100%; diameter = 2-3 nm), in contrast with bulk behavior, which shows immiscibility of Au and Co at room temperature across all composition space. These particles are characterized by transmission electron microscopy and H-1 NMR techniques, as well as inductively coupled plasma mass spectrometry, X-ray photoelectron spectroscopy, and photoluminescence spectroscopy. In particular, H-1 NMR methods allow the simultaneous evaluation of composition-tunable magnetic properties as well as molecular characterization of the colloid, including ligand environment and hydrodynamic diameter. These experiments also demonstrate a route to optimize bimodal imaging modalities, where we identify AuxCoyNP compositions that exhibit both bright NIR emission (2884 M(-1)cm(-1)) as well as some of the highest per-particle T-2 relaxivities (12200 mM(NP)(-1)s(-1)) reported to date for this particle size range.