A synchrotron radiation photoemission study of the adsorption behaviours of ferrocene, Fe (C5H5)2, and nickelocene, Ni (CH5)2, on the Si (111) 2 X 1 surface is reported. The substrate temperature ranged between 80 and 300 K and the exposure range investigated was 1-1000 langmuir. For both species the adsorption is molecular over the entire temperature range, as demonstrated by the one-to-one correspondence between the valence photoemission spectra of the species adsorbed on Si and those of the corresponding free molecules. Exposure of the reacted surfaces saturated with metallocenes to unmonochromatized synchrotron light results in a partial desorption of the species. This result is at variance with the reactivity on Si shown by some Fe, W and Mo carbonyl analogues of the metallocenes, which readily decompose to metal under the same experimental conditions. The higher chemical stability on Si of the metallocenes discussed here with respect to carbonyls is consistent with the corresponding behaviour of the free molecules.