A stable nonlinear adaptive observer is constructed to estimate reactant composition and uncertain model parameters from temperature measurements in an exothermic stirred tank reactor. The construction is based on an adaptive observer canonical formulation proposed in the literature. Only a very limited class of nonlinear systems are known to be transformable to this canonical form, the exothermic reactor with first order irreversible kinetics however reveals itself to be suitable for the transformation. The utility of the adaptive observer is demonstrated by simulations while implementing feedback linearizing control of the reactor in the presence of state and parametric uncertainties.