Standard free energies of formation for the temperature range 298- 1000�K are calculated for C5H5, C5H6, and the biscyclopentadienyl compounds of iron, ruthenium, nickel, and magnesium by combining reported heats of formation at 298�K with calculated ideal-gas thermodynamic properties. The results are used to predict the thermodynamic feasibility of three types of reactions-the formation of biscyclopentadienyl metal compounds, their thermal decomposition, and their reduction with hydrogen. It is shown that the predictions are in agreement with qualitative chemical observations.