Addition of excipients is a common strategy to slow protein aggregation during storage. Excipient effects on the mechanism(s) and temperature (T) dependence of aggregation for a monoclonal antibody solution were tested using size-exclusion chromatography, differential scanning calorimetry (DSC), temperature scanning monomer loss (TSML), and laser light scattering; previous work in buffer-only conditions had shown non-Arrhenius behavior and implicated Fab and/or C_H3 unfolding as a key step in aggregation. Excipients included citrate, amino acid salts (histidine–HCl, arginine–HCl), and polyols (mannitol and glycerol). DSC and TSML showed that Fab, rather than C_H3, unfolding corresponded with the onset of aggregation for each condition. Isothermal incubation at 56.5°C, 40°C, and 2°C–8°C resulted in aggregation, while fragmentation occurred readily at only 40°C. The primary effect of the different excipients appeared to be preferential accumulation/exclusion, affecting the concentrations of partially unfolded monomer key intermediates. In addition, aggregation rates were clearly non-Arrhenius, causing aggregation to dominate over fragmentation at high and low T, and making long-term stability predictions problematic based on commonly employed 40°C conditions. Possible reasons for non-Arrhenius behavior include a strong T-dependence of the Fab unfolding enthalpy and/or a switch from Fab-mediated to Fc-mediated aggregation as one moves from high to low T.