TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - In stage (a), the selective hydrogenation of 1,3-butadiene and isomerization of 1-butene to 2-butenes, the C4 cut, as a liquid phase, is passed over a catalyst comprising Ni, Pd or Pt on a support, at 20 - 200degreesC. and 1 - 5MPa. The space velocity is 0.5 - 10 h-1 and the molar ratio H2 / butadiene is 0.5 - 5. The effluent is mainly 2-butenes and isobutene. In step (b), the distillation column comprises 5 - 200 theoretical plates and operates at 0.3 - 1MPa with a reflux / distillate ratio of 5 - 20. In step (c), total hydrogenation of any residual 1-butene and isobutene in the head effluent from (b) is effected by passing the effluent over a similar catalyst as in (a) and at 20 - 200degreesC. and 1 - 5MPa, a space velocity of 0.5 - 10 h-1 and molar ratio H2 / olefins of 0.5 - 5, to give an effluent mainly of isobutane. In the preferred preparation of the alkylate, the 2-butenes-enriched effluent from (b) and the isobutane effluent from (c) are reacted in an alkylation unit (stage d) to form an isooctane mixture and excess isobutane. The alkylation unit is operated with H2SO4 or HF. Stage (d) is followed by a separation stage (e) by distillation, to remove excess isobutane and recover the high octane alkylate at the base. The excess isobutane is recycled upstream of the hydrogenation zone. EXAMPLE - A C4 cut of the % composition : Propane : 0.27 : isobutane : 20 ; 1-butene : 14.12 ; isobutene : 21.22 ; n-butane : 9.97 ; 2-butenes : 30.80 ; 1,3-butadiene : 1.23 ; C5 : 2.39 ; from a catalytic cracking unit, was continuously introduced in a hydro-isomerization unit at 35313 kg/h and 2 MPa in a fixed bed reactor containing 13 tonnes of a Pd/alumina catalyst pre-sulfurised and reduced in H2. Pure H2 was injected at 70 - 89degreesC. and the effluent was passed to an 80 plate distillation column operating at 0.65 MPa and a head temperature of 44degreesC. with a ratio of reflux flow rate / distillate flow rate of 10. The distillate was mixed with a recycled isobutane flux from the distillation column C2 and passed to the hydrogenation unit at 66405 kg/h. This contained 10 tonnes of a Pd(0.3%) / alumina catalyst pre-reduced in H2 at 100degreesC. Pure H2 was injected at 50 - 96degreesC. and an effluent was obtained of % composition :- Propane : 1.90 ; isobutane : 86.89 ; 1-butene : 0 ; isobutene : 1.07 ; n-butane : 9.99 ; C5 : 0.15 ; This effluent was mixed with the 2-butenes-enriched effluent from the base of the separation column, of % composition :- 1-butene : 0.23 ; isobutene : 1.76 ; n-butane : 19.23 ; 2-butenes : 74.58 ; C5 : 4.20 ; and transformed to high octane index petrol.