Pd(0) nanoparticles with approximately 2 nm diameter, immobilized in 1-n-butyl-3-methylimidazolium hexafluorophosphate ionic liquid, are efficient catalyst precursors for coupling of aryl halides with n-butylacrylate. In situ TEM analysis of the ionic liquid catalytic solution after the catalytic reaction shows the formation of larger nanoparticles ( approximately 6 nm). The palladium content in the organic phase during the arylation reaction was checked by ICP-AS and shows significant metal leaching (up 34%) from the ionic phase to the organic phase at low substrate conversions and drops to 5-8% leaching at higher conversions. These results strongly suggest that the Pd(0) nanoparticles serve as a reservoir of "homogeneous" catalytic active species.