Biophysical properties of antibody-based biopharmaceuticals are a critical part of their release criteria. In this context, finding the appropriate formulation is equally important as optimizing their intrinsic biophysical properties through protein engineering, and both are mutually dependent. Most previous studies have empirically tested the impact of additives on measures of colloidal stability, while mechanistic aspects have usually been limited to only the thermodynamic stability of the protein. Here we emphasize the kinetic impact of additives on the irreversible denaturation steps of immunoglobulins G (IgG) and their antigen-binding fragments (Fabs), as these are the key committed steps preceding aggregation, and thus especially informative in elucidating the molecular parameters of activity loss. We examined the effects of ten additives on the conformational kinetic stability by differential scanning calorimetry (DSC), using a recently developed three-step model containing both reversible and irreversible steps. The data highlight and help to rationalize different effects of the additives on the properties of full-length IgG, analyzed by onset and aggregation temperatures as well as by kinetic parameters derived from our model. Our results further help to explain the observation that stabilizing mutations in the antigen-binding fragment (Fab) significantly affect the kinetic parameters of its thermal denaturation, but not the aggregation properties of the full-length IgGs. We show that the proper analysis of DSC scans for full-length IgGs and their corresponding Fabs not only helps in ranking their stability in different formats and formulations, but provides important mechanistic insights for improving the conformational kinetic stability of IgGs.