The injection of hydrocarbons into the JET Gas Box divertor in ohmic and L-mode discharges has been used to calibrate the intrinsic hydrocarbon erosion yields and to determine the variation of the photon efficiency with the edge ion flux. Methane erosion yields of 5% for CD4 and 3% for CH4 were measured, with no sign of any flux dependence. The erosion yield of ethane/ethene was lower but increased with the edge ion flux (increasing edge plasma density, decreasing edge electron temperature) such that it dominated the molecular carbon source at high density. The methane and ethane photon efficiencies (the number of molecular dissociations per emitted photon) were both not constant, but were found to increase with increasing density (increasing edge ion flux, decreasing edge electron temperature).