Inhibition of terminal respiratory oxidases by nitric oxide (NO) plays important physiological roles in signaling and host defense. Using a bacterial quinol oxidase and mitochondrial cytochrome c oxidase, this chapter describes simple polarographic methods to quantify the kinetic characteristics of inhibition by NO. This chapter points out the inherent pitfalls of both experimental design and data analysis and compares alternative methods. Additionally, it describes a system designed to acquire polarographic and spectral data simultaneously to permit identification of spectral intermediates under defined conditions.