Carbon is the principal JET impurity. The carbon is ionised in the JET Scrape-off-layer (SOL) and flows to the divertor by collisional coupling to the deuterium ions. This "Screening" has been quantified by injecting methane into JET and observing the increased core-plasma contamination. Methane injection mimics the fate of sputtered hydrocarbons and has been performed from various (main chamber and divertor) locations in JET L- and H-mode plasmas. The screening has been measured as a function of the plasma current, density, and applied heating power for L-Mode plasmas. In L-Mode, more carbon penetrates to the core at lower density and lower confinement. The screening is 2 times better for methane puffed from the vessel top than from the mid-plane and 10 times better for methane puffed from the divertor.