Near UV and visible light photodegradation can target therapeutic proteins during manufacturing and storage. While the underlying photodegradation pathways are frequently not well-understood, one important aspect of consideration is the formulation, specifically the formulation buffer. Citrate is a common buffer for biopharmaceutical formulations, which can complex with transition metals, such as Fe(III). In an aqueous solution, the exposure of such complexes to light leads to the formation of the carbon dioxide radical anion ( • CO 2 - ), a powerful reductant. However, few studies have characterized such processes in solid formulations. Here, we show that solid citrate formulations containing Fe(III) lead to the photochemical formation of • CO 2 - , identified through DMPO spin trapping and HPLC-MS/MS analysis. Factors such as buffers, the availability of oxygen, excipients, and manufacturing processes of solid formulations were evaluated for their effect on the formation of • CO 2 - and other radicals such as • OH.