The adsorption of Ca vapor on phenyl-C 61 -butyric acid methyl ester (PCBM) at 300 K has been studied by X-ray photoelectron spectroscopy (XPS), lowenergy He + ion scattering spectroscopy (LEIS), adsorption microcalorimetry, and atomic beam/surface scattering. This interface commonly occurs in some of the highest efficiency organic solar cells. It is found that over 10 nm of the PCBM undergoes aggressive reaction with the Ca vapor to make the Ca carboxylate of PCBM. This thick reacted layer, which was not previously known to be present, lies at the interface between the metallic Ca film and PCBM and is expected to influence charge transfer across that interface during photovoltaic operation. The heat of Ca adsorption is very high below 0.03 ML (800−850 kJ/mol) due to reaction of Ca with impurities. Between 0.05 and 0.4 ML, the heat of adsorption is 624 kJ/mol and nearly constant. This heat is assigned to the reaction of Ca with subsurface methyl ester groups to form the Ca carboxylate of PCBM. This assignment is supported by the shift of the O 1s XPS peak of PCBM toward lower binding energy (BE) due to this reaction with Ca, and the absence of Ca LEIS signal, below 0.4 ML coverage. Conversely, the C 1s XPS peak shifts toward higher BE due to downward band bending. Beyond 0.4 ML, the heat of adsorption decreases nearly exponentially to the sublimation enthalpy of Ca (178 kJ/mol) by 3 ML, attributed to the formation of Ca(solid) nanoparticles on the surface and eventually a continuous Ca film. This model is supported by LEIS. Impinging Ca atoms face a kinetic competition between diffusing subsurface to react with methyl ester groups of PCBM and the formation and growth of three-dimensional Ca clusters on the surface. The total extent of reaction of Ca with subsurface ester groups to make the Ca carboxylate of PCBM is equivalent to ∼14 layers of reacted PCBM molecules or ∼13 nm of reacted depth.