After careful calibration, the work function change induced by CO adsorption on Ru(001) has been used to measure the sticking coefficient s as functions of sample temperature (between 100 and 400 K) and coverage, with the gas temperature always at about 295 K. The main results are: At θ=0, the sticking coefficient is independent of sample temperature between 100 and 400 K, at s0=0.7 for 0<θ<0.2. s drops proportional to (1–θ) for Ts≳200 K, and more gradually below 200 K; at 100 K, s drops slowly up to θ=0.5. Above θ=0.23 a sharp decrease of s sets in for Ts≳120 K; a second plateau becomes obvious around θ∼0.35 for Ts<360 K. Close to saturation, s drops to below 10−2. These results are discussed in terms of a precursor model with strong effects of the chemisorbed layer on the properties of the precursor. A critical evaluation of the formal precursor states involved suggests that classical precursor states exist only at appreciable coverage and are defined by the lateral and vertical interactions, while at low coverage the collisional dynamics are determined for sticking of molecules.