This work presents nonlinear observer designs for the catalytic partial oxidation (CPO) and the water gas shift (WGS) reactors in fuel cell power systems. The observers make use of available temperature and total pressure measurements and estimate the mole fractions of each species in the reactors. Estimation of H/sub 2/ and CO fractions is particularly useful for monitoring and control of the fuel processor. An advantage of our designs is that they are based on the reaction invariants and do not rely on knowledge of reaction rate expressions.