Adsorption of CO and O2 on Pd(111) has been studied with the molecular beam technique. Absolute sticking coefficients for CO and O 2 have been determined as a function of coverage, angle of incidence, and substrate temperature. Angular distributions of the scattered molecules show that the probability for capture in the precursor state is unity for CO at low coverages and for all substrate temperatures investigated and 0.5 for O 2 at low coverages independent of the substrate temperature. Modulated beam experiments show that Edes. for CO is 32 kcal/mole at low coverages. Phase sensitive detection and waveform analysis show that at high CO coverages on the impurity free surface and over a wide range of coverages on a carbon contaminated surface, three distinct CO species are present on the surface. © 1978 American Institute of Physics.