The influence of potassium species on a Co based Fischer-Tropsch catalyst was investigated using an aerosol deposition technique. This way of poisoning the catalyst was chosen to simulate the actual potassium behaviour during the biomass to liquid (BTL) process utilizing gasification followed by fuel synthesis. A reference catalyst was poisoned with three levels of potassium and the samples were characterized and tested for the Fischer-Tropsch reaction under industrially relevant conditions. None of the conventional characterization techniques applied (H2 Chemisorption, BET, TPR) divulged any difference between poisoned and unpoisoned samples, whereas the activity measurements showed a dramatic drop in activity following potassium deposition. The results are compared to previous results where incipient wetness impregnation was used as the method of potassium deposition. The effect of potassium is quite similar in the two cases, indicating that irrespective of how potassium is introduced it will end up in the same form and on the same location on the active surface. This indicates that potassium is mobile under FTS conditions, and that potassium species are able to migrate to sites of particular relevance for the FT reaction.