A concentration slip model to describe the rarefied gas effect on the species transport in microscale chemical reactors was derived from the approximate solution of the Boltzmann equation. The present model is more general and recovers the existing models in the limiting cases. The analytical results showed that the concentration slip is governed by the reaction induced concentration slip (RIC) and the temperature slip induced concentration slip (TIC). The magnitude of RIC slip is proportional to the product of the Damköhler number and Knudsen number. The impact of concentration slips was examined using detailed chemistry of methane in a two-dimensional microscale catalytic reactor. The results showed that the impact of reaction induced concentration slip strongly depends on the Damköhler number, the Knudsen number and the surface accommodation coefficient. It was found that the TIC slip had a strong effect on the fuel oxidation rates and the RIC slip dramatically changed the mass fraction distributions of radicals especially when the mass accommodation coefficients were far less than unity.