NOVELTY - Alkylation of isobutane comprises: (a) reacting an isobutane-rich feed (3) with a light-olefin-rich feed (2); (b) passing the effluent to a fractionation column (6); (c) recycling the isobutane-rich fraction (9) to the alkylation reactor; and (d) purifying the n-butane-rich fraction (8) to reduce its C5+ content, passing it through an isomerization reactor (16) to convert n-butane to isobutane, and recycling it to the fractionation column.    USE - The alkylate product is useful as an aviation fuel additive or high-octane gasoline blending stock.    ADVANTAGE - Step (d) allows the amount of fresh isobutane required to maintain a given alkylate yield to be reduced, e.g. by 21.75% compared with a similar process with C5+ removal but no isomerization, and by 6.55% compared with a similar process with isomerization but no C5+ removal.    DETAILED DESCRIPTION - Alkylation of isobutane with olefins comprises:    (a) reacting an isobutane-rich feed (3) with a light-olefin-rich feed (2) in an alkylation reactor;    (b) passing the effluent to a fractionation column (6) to obtain an alkylate-rich fraction (7), an n-butane-rich fraction (8) and an isobutane-rich fraction (9);    (c) recycling the isobutane-rich fraction to the alkylation reactor; and    (d) purifying the n-butane-rich fraction to reduce its C5+ content to 5 wt.% or less, passing it through an isomerization reactor (16) to convert n-butane to isobutane, and recycling it to the fractionation column.    An INDEPENDENT CLAIM is also included for apparatus for alkylating isobutane with olefins, comprising: an alkylation reactor (1); a fractionation column (6) with a bottom outlet for alkylate, a middle outlet for n-butane and a top outlet for isobutane; an isobutane recycle line (9); a rectifier (12) for removing C5+ hydrocarbons from the n-butane fraction; an isomerization reactor (16) for converting n-butane to isobutane; and an isomerizate recycle line (21).    DESCRIPTION OF DRAWING(S) - The drawing shows an apparatus for carrying out the process.    alkylation reactor (1)    olefin feed (2)    isobutane feed (3)    fractionation column (6)    alkylate fraction (7)    n-butane fraction (8)    isobutane fraction (9)    rectifier (12)    isomerization reactor (16)    isomerizate recycle line. (21)