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Gold nanoparticles (AuNPs) are now being used in such areas as diagnostics, drug delivery, and biological sensing. In these applications, AuNPs are frequently exposed to biological fluids. These fluids contain many different proteins, any of which may interfere with the intended function of the nanoparticle. In this work, we examine the thermodynamic consequences of proteinnanoparticle binding using a combined spectroscopic and calorimetric approach. We monitored binding using UV-Vis spectroscopy, differential scanning calorimetry (DSC), and isothermal titration calorimetry (ITC). Six proteins were studied based on their differing chemical properties, and both 15 nm and 30 nm citrate-coated AuNPs were investigated. We interpreted the UV-Vis data using two different models: the commonly-used Langmuir isotherm model and a more complex mass transport model. Both models can be used to determine Kd values for the 30 nm AuNP data; however, the mass transport model is more appropriate for 15 nm AuNPs. This is because, when fitting the Langmuir model, it is commonly assumed that most proteins are not surface-associated, and this assumption fails for 15 nm AuNPs. The DSC thermograms show two transitions for a globular protein adsorbed to a 15 nm AuNP: one high-temperature transition that is similar to global protein unfolding (68 C), and one low-temperature transition that may correspond to unfolding at the surface (56 C). Conversely, ITC experiments show no net heat of adsorption for GB3, even at high protein/AuNP concentrations. Together, the spectroscopic and calorimetric data suggest a complex, multi-step process for protein-nanoparticle adsorption. Moreover, for the proteins studied, both AuNP curvature and protein chemistry contribute to protein adsorption, with proteins generally binding more weakly to the larger nanoparticles. In the future, this work may lead to principles for improving the design of AuNPbased therapeutics and sensors.