The activity of a supported catalyst containing copper, potassium and molybdenum was studied. Model experiments revealed that chlorine plays an essential role in the activity of this catalyst. It was shown that the active species of the catalyst probably consist of copper chloride compounds. Deactivation of the catalyst was studied, and was found to be more pronounced for a poor contact between soot and catalyst compared with a tight contact. Deactivation rates were found to be low, which was tentatively suggested to be caused by loss of active species that are formed by solid-solid reactions (e.g. KCl + CuMoO4 --> K-molybdates + Cu-(oxy)chlorides) which are slow as a result of low solid-state diffusion rates. A Cu/K/Mo-catalyst as a coating on small segments of a wall flow monolith downstream of a diesel engine showed a relatively low activity. Besides, the catalyst was found to deactivate rather fast, which corroborated the outcome of the above mentioned model study. As a result, the feasibility of this Cu/K/Mo-catalyst for use in practical applications is low due to a progressive loss of catalytic material by high vapour pressures of active components formed by solid-solid reactions of less volatile compounds present in the catalyst.