Freeze-thawing is a potentially damaging stress to which therapeutic proteins can be exposed deliberately during storage of bulk drug substance, and accidentally because of mishandling of commercial product during shipping and/or storage. The primary route of degradation induced by freeze-thawing is protein aggregation. We studied the effects of freeze-thawing on aggregation of an IgG2 monoclonal antibody, examining solution conditions (pH, and the presence or absence of 150 mM KCl), protein concentration, cooling and warming rates, and container type and material. In addition, we determined the effect of pH and KCl on protein tertiary structure and thermal stability with second derivative UV spectroscopy. In general, aggregation of the antibody during freeze-thawing increased with decreasing pH, which correlated well with Tm values. Aggregation was most prevalent at pH 3 and 4, with potential mechanisms involving both the formation of aggregation-prone conformational states as well as adsorption to and denaturation at various interfaces. Although all the parameters examined demonstrated some effect on the formation of soluble aggregates, the effect of container material was especially pronounced. Samples stressed in plastic or glass containers contained low amounts of aggregate. Storage in Teflon or commercial freezing containers, however, led to significantly higher levels of aggregate formation.