We study the preparation and capping of silver nanoparticles by several unsaturated long-chain carboxylates. UV-visible and FTIR spectroscopy and high-resolution electron microscopy are used to characterize the effect of the chain length, its configuration, and the degree of unsaturation on the size distribution of the nanoparticles. Langmuir layers and Langmuir-Blodgett films are used to study the adsorption of these carboxylates on the particles. We find that unsaturated carboxylates in the cis configuration are useful stabilizers for the control of particle size and its surface properties.