Abstract. Polysorbate 80 is widely used in protein formulations to protect protein against agitationinduced aggregation. In this study, we address concerns about residual peroxide present in Polysorbate 80 on protein stability. Residual peroxide may oxidize active pharmaceutical ingredients leading to reduced stability and may ultimately lead to lower potency and efficacy. The effect of Polysorbate 80 concentration on thermal and photostability of monoclonal antibody of the IgG1 subclass (MAb1) was evaluated at Polysorbate 80 concentrations ranging from 0.00% to 1.00% (w/v). MAb1 samples at 5 mg/ mL with various Polysorbate 80 concentrations were subjected to accelerated thermal stress by incubation at 25°C, 40°C, and 50°C for a period of 4 weeks and light stress per ICH guideline Q1B, option 1. Our results show that Polysorbate 80 concentration of 1.00% (w/v) adversely affected thermal and photostability of MAb1. This study demonstrates the importance of carefully choosing Polysorbate 80 concentration in protein formulations to prevent destabilizing effect of Polysorbate 80 on thermal and photostability.