Estimates of erosion, deposition and H-isotope retention in JET from previous divertor campaigns have relied on analysis of in-vessel components removed at shutdowns. The components analysed have also provided an incomplete coverage of the vessel. In 2004 new diagnostics are being installed to give a more complete picture (such as smart tiles) and to provide some time resolution. The latter includes further quartz micro-balances, following the successful operation of a prototype in 2002-4, which will also have temperature control. Other diagnostics include rotating collectors and deposition monitors. Units are also being installed to provide information on mirrors for ITER.