The electrical properties of insulators formed by focused ion beam induced deposition of various siloxane precursor gases have been compared. Leakage current and breakdown field have been measured by forming metal-insulator-metal structures. It was found that. the focused ion beam induced deposition of metal on top of the insulator can substantially degrade the quality of the insulator. We found that the resistivity of the insulator material depends on the deposition yield (e.g., the amount of Ga implantation) as well as on the chemical nature of the precursor gas. From the precursor gases studied, the new compound pentamethylcyclopentasiloxane shows the best performance. Compared to the commercially used tetramethylcyclotetrasiloxane compound, an improvement in resistivity by two orders of magnitude (similar to 8 x 10(11) versus similar to 6 x 10(9) Omega cm) and a factor of about 1.5 in breakdown field (650 vs 440 V/mu m) could be achieved.