Employing a molar excess of carbon dioxide (P-c = 71.8 bar; T-c = 31.1 degrees C), supercritical 1-butene/ isobutane alkylation is performed at temperatures lower than the critical temperature of isobutane (<135 degrees C), resulting in virtually steady alkylate (trimethylpentanes and dimethylhexanes) production on both microporous zeolitic (H-USY) and mesoporous solid acid (sulfated zirconia) catalysts for experimental durations of up to nearly 2 days. At a space velocity of 0.25 g of 1-butene/g of catalyst/h, a feed CO2/isobutane/olefin ratio of 86:8:1, 50 degrees C, and 155 bar, roughly 5% alkylate yield (alkylates/C5+) and 20% butenes conversion are observed at steady state. The ability of the carbon dioxide based supercritical reaction mixtures to mitigate coking and thereby to maintain better pore accessibilities is also evident from the narrow product spectrum (confined to C-8＇s), the lighter color of the spent catalyst samples, and relatively low surface-area and pore-volume losses (<25%) in the spent catalysts. For identical weight hourly l-butene space-velocity and feed isobutane/olefin. ratios, the alkylate formation declines continuously with time when the reaction is carried out without employing carbon dioxide. At the high temperatures (>135 degrees C) required for supercritical operation without carbon dioxide, cracking and coking reactions are dominant as inferred from the rather wide product spectrum and extensive surface area/pore volume losses (up to 90%) in the spent catalysts. The carbon dioxide based, fixed-bed, solid acid alkylation process shows promise as an environmentally safer alternative to conventional alkylation that employs liquid acids.