Abstract Sulfur mustard (mustard gas) and arsenicals such as lewisite are major threats as chemical warfare or terrorist agents. Despite different chemical structures, both types of agents lead to similar skin damage with large blisters and wounds that heal slowly, raising the specter of shared pathogenic mechanisms. In this proposal, we will investigate the role of oxidant skin injury as a shared mechanism in mustard and arsenical vesicant injury, employing nitrogen mustard and phenylarsine oxide as surrogates for sulfur mustard and lewisite, respectively. Although major advances have been made in vesicant research, we still do not have effective counter agents. This may be in part due to lack of full understanding of the pathogenesis of vesicants. The overarching hypothesis of this proposal is that defining the early effects of vesicants in vivo at subcellular and single cell levels will refine our understanding of the role of oxidative stress in vesicant skin injury and lead to new therapeutic strategies. We will also test the hypothesis that cobinamide, a highly effective and versatile antioxidant that neutralizes both reactive oxygen and nitrogen species, will be effective as a counter-agent and as means to understand better the role of oxidative stress in nitrogen mustard- and phenylarsine oxide-induced vesicant injury. The specific aims are: 1) To understand the initiation of nitrogen mustard- and phenylarsine oxide-induced skin injury at a single cell level. We will use in vivo fluorescence lifetime imaging (FLIM) and single cell RNA sequencing to gain a new view on the initiation of vesicant injury and the role of oxidative stress in the pathogenesis. 2) To define the efficacy of cobinamide against nitrogen mustard- and phenylarsine oxide-induced vesicant skin injury. We will test the efficacy of cobinamide against nitrogen mustard- and phenylarsine oxide-induced vesicant skin injury in mice. Due to differences between mouse and human skin, we will also investigate vesicant mechanisms in transplanted human skin. These studies are significant and innovative because they use state of the art tools to investigate a gap in our knowledge about the role of oxidative stress in the initiation of mustard and arsenical vesicant injury and because they test for the first time cobinamide, a highly effective antioxidant that is bifunctional for reactive oxygen and nitrogen species.