First proposed in the early 1780s, Richard Kirwan's phlogiston theory was the most successful enunciation of the English pneumatic approach to phlogiston. Phlogiston was identified with a material substance, inflammable air. In this paper, I explore the nature of Kirwan's theory, its success in the mid-1780's, the unprecedented collective attack on Kirwan's Essay on Phlogiston by Lavoisier and his colleagues, and Kirwan's ultimate abandonment of phlogistic explanation.