Gas-phase reactions of acylium ions with alpha, beta-unsaturated carbonyl compounds were investigated using pentaquadrupole multiple-stage mass spectrometry. With acrolein and metacrolein, CH3-C+=O, CH2=CH-C+=O, C6H5-C+=O, and (CH3)(2)N-C+=O react to variable extents by mono and double polar [4+2(+)] Diels-Alder cycloaddition. With ethyl vinyl ketone, CH3-C+=O reacts exclusively by proton transfer and C6H5-C+=O forms only the mono cycloadduct whereas CH2=CH-C+=O and (CH3)(2)N-C+=O reacts to great extents by mono and double cycloaddition. The positively charged acylium ions are activated O-heterodienophiles, and mono cycloaddition occurs readily across their C+=O bonds to form resonance-stabilized 1,3-dioxinylium ions which, upon collisional activation, dissociate predominantly by retro-addition. The mono cycloadducts are also dienophiles activated by resonance-stabilized and chemically inert 1,3-dioxonium ion groups, hence they undergo a second cycloaddition across their polarized C=C ring double bonds. O-18 labeling and characteristic dissociations displayed by the double cycloadducts indicate the site and regioselectivity of double cycloaddition, which are corroborated by Becke3LYP/6-311++G(d,p) calculations. Most double cycloadducts dissociate by the loss of a RCO2COR1 molecule and by a pathway that reforms the acylium ion directly. The double cycloadduct of the thioacylium ion (CH3)(2)N-C+=S with acrolein dissociates to (CH3)(2)N-C+=O in a sulfur-by-oxygen replacement process intermediated by the cyclic monoadduct. The double cycloaddition can be viewed as a charge-remote type of polar [4+2(+)] Diels-Alder cycloaddition reaction. Copyright (C) 2001 John Wiley Sons, Ltd.372146154