Chemical erosion of carbon can be described as function of the incident ion flux, the ion energy and the surface temperature. Spectroscopy on the hydrocarbon break-up products CH and C2 is applied to determine in-situ hydrocarbon fluxes and quantify the chemical erosion by means of hydrocarbon injection. Present-day knowledge on critical issues concerning the erosion yield and its determination is presented: Effective conversion factors are measured in TEXTOR for different hydrocarbon species and compared with calculations from HYDKIN. The underlying database for the methane break-up, used in different erosion/deposition models, has been put to test. The chemical erosion yield related to higher hydrocarbons is determined to be lower in the JET outer divertor than measured in previous experiments. In ASDEX Upgrade and DIII-D rst in-situ calibrations of hydrocarbon fluxes in the detached outer divertor are performed.