The interactions of an organometallic precursor to Ru thin films, bis(ethylcyclopentadienyl)ruthenium, (C2H5C 5H4)2Ru, with an amorphous Al2O 3 substrate were probed using tools of ultra-high vacuum surface science. The precursor adsorbs and desorbs with no decomposition on Al 2O3 that is bare, or covered with patches of O-saturated Ru. In the absence of pre-existing Ru sites, there was no reaction up to 950 K with or without co-dosed O2 at pressures up to 10-6 torr. Film-forming reactions do occur under UHV dosing conditions when Ru sites and chemisorbed oxygen are both present. The distribution of chemical states of surface oxygen changes with temperature and influences reaction kinetics and film growth.