A differentially pumped, capillary array molecular beam source is used to study the reversible adsorption of CO on CO/Pd(111) [Theta (CO)=0.55] and vinylidene/Pd(111) [Theta (vinylidene)=1.0] at 300 K. Differentially pumping allows the beam to equilibrate rapidly (in similar to2 s) while maintaining good beam uniformity. The isosteric heat of adsorption of reversibly held CO on a surface precovered with 0.55 monolayers of chemisorbed CO is 5.6 +/-0.2 kcal/mol at low excess coverages but decreases linearly with coverage so that an additional 2% of a monolayer of CO reduces the isosteric heat of adsorption to 3.0 +/-0.2 kcal/mol. CO adsorbs reversibly on vinylidene-saturated Pd(111) with an isosteric heat of adsorption of 1.0 +/-0.5 kcal/mol for coverages up to similar to1% of a monolayer of CO. Infrared spectra of CO on vinylidene-covered Pd(111) at higher pressures (several Torrs) reveal that CO adsorbs on the metal surface. Assuming that the heat of adsorption of CO on vinylidene-covered Pd(111) decreases with CO coverage at higher coverages shows that the low-coverage data measured using molecular beam methods are consistent with the isotherm measured using infrared spectroscopy at higher pressures.