The invention provides a process for evaluating the effect of a refinery feedstock on a refinery process, said process comprising: (i) providing a refinery feedstock; (ii) treating said refinery feedstock to produce a plurality of fractions each representative of the typical feedstock for the refinery process of interest, said plurality of fractions comprise at least two fractions with different properties; (iii) treating each of the plurality of fractions under experimental conditions representative of those in the refinery process, said treatments being carved out in an essentially parallel manner; and (iv) determining one or more performance criteria in respect of each fraction for the refinery process.