NOVELTY - The main products from the process are isobutane and 2-butenes which can be reacted in an alkylation unit to form isooctane mixtures with a high octane index.    USE - An alkylate consisting of an isooctane mixture is produced which can be used as a constituent for high octane petrols.    ADVANTAGE - The 1,3-butadiene in the original feedstock is eliminated by hydrogenation, which obviates undesirable side reactions in the alkylation operation (formation of sludges etc.). The transformation of 1-butene to 2-butenes enables their easier separation from isobutene and thus allows hydrogenation mainly of isobutene to isobutane. The use of 2-butenes in the alkylate olefinic charge gives products of improved octane index, whether by the H2SO4 or the HF process. Typical values for HF-alkylated products were : IOR (octane index research) : 1-butene : 90 - 91 ; 2-butenes : 96 - 97 ; isobutene : 94 - 95 ; IOM (octane number motor) : 1-butene : 88 - 89 ; 2-butenes : 92 - 93 ; isobutene : 91 - 92 ; Excess isobutane from the alkylation zone can be recycled.    DETAILED DESCRIPTION - A new process for increasing the isobutane content of a C4 cut from a catalytic or vapor cracking unit, which comprises mainly n-butane, isobutane, isobutene, 1-butene and 2-butenes, also including a small amount of 1,3-butadiene. The stages include :    (a) Hydroisomerization of the C4 cut to form a mixture mainly of 2-butenes, isobutene and isobutane ;    (b) Separation by distillation of the hydro-isomerized effluent into a base effluent enriched in 2-butenes and a head effluent enriched in isobutane and isobutene ; and    (c) Hydrogenation of the isobutene/isobutane effluent in a hydrogenation zone.    INDEPENDENT CLAIMS are also included for:    (1) The isobutane-enriched C4 cut;    (2) Preparation of an alkylate with improved octane index; and    (3) The isooctane mixture obtained.    DESCRIPTION OF DRAWING(S) - DRAWING DESCRIPTION - The figure 1 illustrates an installation    hydro-isomerization zone (R1)    distillation separation column (C1)    hydrogenation zone (R2)    alkylation zone (R3)    isobutane/alkylate separation column (C2)