A study of the weathering properties of Gullfaks A blend and Gullfaks C blend crude oils, has been performed at 5°C and 13°C in laboratory scale. The data are used as input to SINTEF Oil Weathering Modell to predict the behaviour of the Gullfaks A and C blends in a spill situation at sea. The weathering properties are discussed in relation to oil spill response, mechanical recovery and chemical dispersion. Mechanical recovery:Formation of viscous emulsions reduces problems with boom leakage in a recovery operation. Boom leakage, due to low viscosity (< 1 000 mPas = cP), will only be expected the first 4 - 5 hours after release for Gullfaks A blend at 10 m/s wind. Gullfaks C blend will cause this problem approx. 3 hours at winter temperature and 10 m/s wind. This problem time will extend with lower sea state for both oils. Studies at SINTEF have shown that weir skimmers operated by NOFO (Transrec) and adsorption skimmers may have reduced collection effect at viscosities above 20 000 mPas. For Gullfaks A this viscosity limit is achieved after approx. 5 days, both at winter and summer conditions, at high wind speed (15 m/s), and for Gullfaks C much earlier, after approx. 1- 2 days at the same conditions. Use of dispersants:Gullfaks A has a good potential for use of chemical dispersants, even at winter conditions, where the effect will be reduced after approx. 6 hours after the spill at 10 m/s wind. However, at summer conditions the time window will be somewhat extended, and dispersants may be effective the first 12 hours in calm weather conditions (5 m/s wind). Gullfaks A will be chemically dispersible at least the first 5 days after the spill. Gullfaks C, however, will not be chemically dispersible after 4 days at winter temperature and 5 m/s wind and have reduced chemical dispersibility after 10 hours at the same weather conditions.