The search for alternative fuels has lead to a number of possibilities. The most promising alternative fuels for the short term are drop-in fuels such as synthetic fuel and hydrotreated renewable jet. These fuels are similar to Jet A-1 but some differences are present. Synthetic fuels are produced using a process that results in a fuel without trace elements and almost no aromatics. Furthermore, synthetic fuels have a higher energetic content and lower gravimetric density than Jet A-1. These differences cause several effects when considering the use of synthetic fuel in aircraft. A performance model is used to show that the payload-range performance is changed and that an efficiency gain is achieved on the fuel consumption for a regular flight. Measurement of the soot emissions for several blends of synthetic fuel with Jet A-1 show that increasing the amount of synthetic fuel leads to significant reductions in soot emissions. Reductions of 50 to 70% in particle mass emitted can be reached by using 50% synthetic fuel. This might reduce the amount of contrails and aircraft induced cirrus clouds and seriously increase local air quality around airports.