The European Union aims to make existing and new mobility structures more efficient and more ecological. Approaches to achieve this include electromobility, hybridisation or alternative fuels. In the project LeanDeR described in this article, the environmental impact of the alternative fuel natural gas was investigated for use with port handling equipment. This involved a terminal tractor powered purely by natural gas and a reach stacker fuelled by a combination of diesel and natural gas. For both machines, the conventional diesel drive was used as a benchmark for comparison with the aforementioned alternative drives. During a field test, the specific emissions of the exhaust gas components CO2, CO, NOx, CH4 and SO 2 were evaluated. The results showed that, in the case of natural gas operation, the specific CO2 emissions of the terminal tractor could be reduced by approximately 10% and the specific CO and NOx emissions by approximately 50% compared to the specific emissions in diesel operation, although methane slip occurred. In addition, particulate emissions were significantly lower in natural gas operation. In the case of the reach stacker, the specific CH4 emissions in mixed operation increased noticeably with the increasing substitution share of natural gas, which can neutralise possible savings in terms of specific CO 2 emissions. In mixed operation, the specific NOx emission decreased with an increasing substitution rate of natural gas. Nevertheless, an increase in the substitution rate led to an increase in specific CO emissions. This behaviour can possibly be attributed to the fact that the engine control unit was optimized for diesel operation. It is shown that the practical use of liquefied natural gas as an alternative fuel with the associated beneficial environmental effects is possible. It is also shown that ideally a pure natural gas engine should be used. In principle, the conversion of existing diesel engines to mixedfuel operation is possible and has potential. However, there is a risk that many advantages will be cancelled out, for example due to missing technical adjustments such as the engine control.