Carboxylative cyclization of propargyl alcohols in supercritical carbon dioxide (scCO2) containing P(n-C4H9)3 as acatalyst proceeded smoothly to give α-alkylidene-1,3-dioxolan-2-ones. Internal propargyl alcohols afforded Z-alkyl-idene cyclic carbonates exclusively. CO2 incorporation wasmarkedly promoted under supercritical conditions, possiblydue to the facile formation of a putative P(n-C4H9)3−CO2adduct as a key intermediate.