A tert-butyl lithium modified Cr(II)/SiO2 catalyst has been prepared and tested in slurry polymerisation of ethylene with and without hydrogen present. Modification with t-butyl lithium results in a catalyst which produces low molecular weight I-olefins in situ that later are co-polymerised into the polymer chain and thus make LLDPE with densities in the range 0.92-0.94 g/cm(3). Polymerisations with hydrogen present leads to a drop in polymerisation activity and increased chain branching. By fractionally separating the gaseous, liquid and solid products of the reactor after the polymerisation complete analyses of the products have been carried out, Polymerisations with deuterium present give products such as HD and 1,2-dideuteroethane in the gaseous phase, while a number of deuterated l-olefins are observed in the liquid phase. The products have been analysed by GC, MS, IR and NMR spectroscopy. The differently deuterated products formed have been used to deduce the mechanisms for hydrogen/deuterium consumption.