Unleaded blend compositions, as well as formulated gasolines containing them have a Motor Octane Number (MON) of at least 80 comprising either: (i) component (a) at least 5% (by volume of the total composition) of at least one hydrocarbon having the following formula I R--CH.sub.2--CH(CH.sub.3)--C(CH.sub.3).sub.2--CH.sub.3 I wherein R is hydrogen or methyl, especially triptane, or (ii) at least 2% of component (a'), which is at least one branched chain alkane of MON value of at least 90 and of boiling point 15-160.degree. C. or a substantially aliphatic hydrocarbon refinery stream, of MON value of at least 85, at least 70% in total of said stream being branched chain alkanes, said stream being obtainable or obtained by distillation from a refinery material as a cut having Initial Boiling Point of at least 15.degree. C. and Final Boiling Point of at most 160.degree. C., said Boiling Points being measured according to ASTMD2892, or (iii) at least 10% of component (a''), which is at least one branched chain alkane of 8-12 carbons with at least 4 methyl or ethyl branches The components (a), (a') and (a'') give rise to reduced emissions to the composition or gasoline on combustion.