The ability of surfactant aggregate-H2O interfaces to control the regioselectivity of Diels-Alder reactions has been investigated. Cycloadditions of surfactant 1,3-dienes 2-[[3-(dimethyldodecylsilyl)-1,3-butadien-2-yl]thio]-N,N,N-trimethyl-1-ethanaminium iodide (1a) and 6-[[3-(dimethyloctylsilyl)-1,3-butadien-2-yl]thio]-N,N,N-trimethyl-1-hexanaminium iodide (1b) with surfactant dienophiles (E)-2-[[[2-(dodecoxycarbonyl)-ethenyl]carbonyl]oxy]-N,N,N-trimethyl-1-ethanaminium iodide (2a) and (E)-6-[[[2-(octoxycarbonyl)ethenyl] carbonyl]oxyl-N,N,N-trimethyl- l-hexanaminium bromide (2b) within their aqueous mixed micelles have been performed at 25(35) degrees C. The cycloaddition of 1a and 2a gave a 30:1 ratio of trans-1-[(2-trimethylammonio)-ethylthio]-2-(dimethyldodecylsilyl)-4-(dodecoxycarbonyl)-5-[(2-trimethylammonio)ethoxycarbonyl]-1-cyclohexene dihalide (15a) and trans-1-[(2-trimethylammonio)ethylthio]-2-(dimtrimethylammonio) ethoxycarbonyl]-5-(dodecoxycarbonyl)-1-cyclohexene dihalide (16a), respectively, and that of Ib and 2b a 6.6:1 ratio of trans-1-[(6-trimethylammonio)hexylthio]-2-(dimethyloctylsilyl)-4-(octoxycarbonyl)5-[6-(trimethylammonio)hexoxycarbonyl]- 1-cyclohexene dihalide (15b) and trans-1-[(6-trimethylammonio)hexylthio]-2-(dimethyloctylsilyl)-4-[6-( l-cyclohexene dihalide (16b), respectively. The excess of 15 over 16 is consistent with the reaction of 1 and 2 within mixed aggregates in their preferred orientations at the aggregate-H2O interface. The greater regioselectivity obtained in the reaction of la and 2a is ascribed to the shorter tether between their reactive functional groups and quaternary ammonium headgroups. A monolayer study of 15a and 16a was also performed.