In his An Essay on Phlogiston and trhe Constitutions of Acids (1784), Kirwan provided an overview of the phlogiston theory, which he supported. Kirwan encountered opposition from the circle of French chemists surrounding Antoine Laurent Lavoisier, which he referred to as "anti-phlogistic".
In 1799, with An Essay on the Analysis of Mineral Waters (which is usually regarded as his magnum opus), he followed up on the work of T. Bergman, providing one of the first summary overviews of analytical chemistry.