In the JET Mark I and Mark II divertors the in-situ chemical sputtering yield of the carbon target plates has been studied under a wide range of conditions. To evaluate the chemical yield, the incident hydrogenic flux is directly evaluated from Langmuir probe measurements while the methane production rate is inferred from the CD/CH molecular band emission calibrated using methane injection experiments. It is found that the chemical sputtering yield reduces at high flux densities >2x1022m-2s-1) and low impact energies (<50eV). In the Mark II divertor both the absolute yield and the flux dependence depend upon the tile temperature with Ychem ∝ Γ-0.4 at 450K and Ychem ∝ Γ -0.7 at 360K. In the Mark I divertor the yield exhibits an even stronger flux dependence (Ychem ∝ Γ-1.25) which is consistent with the relatively low tile temperature of 300K. However, the increased flux densities are associated with reduced impact energies which may also contribute to the yield reduction. The chemical yield is also shown to depend strongly on the isotope mass (Ychem ∝ mion) and is consistent with thermal reactions enhanced by radiation damage.