Au(I) and Au(III) salts (NaAuCl4 and KAuCN2 respectively) were found to be active catalysts for the cyclopropa nation of alkenes with ethyldiazoacetate, in many cases affording high yields of cyclopropane-carboxylates. But these gold salts rapidly decomposed and agglomerated when working with organic solvents, resulting in less active and less selective gold metal particles. With ionic liquids (ILs) as solvents, the gold catalysts were stabilized, especially Au(l), and products and catalyst separation and recycling Could be achieved. In both cases, NaAuCl4 and KAuCN2 underwent reductive transformation to Au(0) to afford gold nanoparticles, which remained stabilized in the IL, behaving as an authentic metal nanoparticle reservoir. It was found that cyclopropanation and aromatic addition were sensitive to gold particle size, whereas the formation of fumarate and maleate esters was insensitive to gold particle size. (c) 2008 Elsevier Inc. All rights reserved.