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—CH2—CH(CH3)—C(CH3)2—CH3  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° 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° C. and Final Boiling Point of at most 160° 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.