Soft-x-ray photoelectron spectroscopy (SXPS) is utilized to study the coadsorption of K and PF3 on Ru(0001) at 90 and 300 K. In the absence of K, PF3 adsorbs molecularly at both temperatures. In the presence of a fractional monolayer of K, initially PF3 completely dissociates resulting in the formation of adsorbed KF and P species. As the surface is further exposed to PF3, some of the PF3 molecules adsorb via partial dissociation, resulting in the formation of PF and PF2. This process continues until all the K has reacted. At 300 K, a fraction of the incoming PF3 molecules react with the adsorbed KF and form a species which is tentatively identified as KPF6. The data show that surface chemistry is different at the two temperatures, as some of the chemical reaction channels occurring at 300 K are blocked at 90 K. The reduced surface mobility of the incident PF3 molecules at 90 K adversely affects the probability of PF3 and KF interactions, which, in tum, causes the concentration of adsorbed PF3 relative to P to be larger at lower temperatures.