The effects of composition of heavy oils derived from petroleum and biomass, on their response to cracking over catalysts of various composition were investigated. The contribution to the conversion from different types of cracking was estimated and the effect of temperature on the product distribution was studied. Heavy fractions of Wilmington crude contained more aromatics and polars compared with a conventional HVGO. The conversion of the Wilmington fractions increased with boiling point range. The zeolitic contribution to the conversion decreased while the matrix contribution remained constant and the contribution from thermal cracking increased. The low H/C-ratio of FCC feed derived from liquefied biomass led to low conversion and poor gasoline selectivity. Addition of alumina to the matrix resulted in a catalyst more active for heavy oil cracking but with a poor selectivity. Alumina-montmorillonite catalysts showed activities for heavy oil cracking comparable to that of a conventional, zeolite based, cracking catalyst. Effects of matrix composition and zeolite type on the heavy oil cracking performance are discussed.