In the present work very-high-n Rydberg-atoms (n∼100−n∼400) are used to probe electron-molecule collisions at ultra-low electron energies. Based on the essentially-free-electron model, for sufficiently high n, Rydberg atom-molecule collisions can be described in terms of a binary interaction between the target molecule and the essentially-free Rydberg electron. Since the average kinetic energy of the Rydberg electron is ultra-low (∼85μeV−∼1.4meV), analysis of the very-high-n Rydberg-atom collision data can provide information on electron-molecule scattering at electron energies corresponding to electron temperatures of only $\sim1\sp\circ$K, which are far below those accessible using any alternate approach. Rate constants for destruction of very-high-n Rydberg atoms in collisions with various target molecules have been measured. This study focuses on Rydberg electron transfer to an electron-attaching molecule which results in negative ion formation via the reactions $$\rm K({\it np/}) + XY \to K\sp+ + (XY)\sp{-}\ or\ \to K\sp+ + X\sp- + Y\eqno(1)$$and on rotational energy transfer from polar molecules which leads to Rydberg atom ionization $$\rm K({\it np/}) + XY({\it J/}) \to K\sp+ + {\it e}\sp- + XY({\it J/}-1)\eqno(2)$$ The n dependence of the rate constants for Rydberg-atom destruction depends on the reactions involved. In reaction (1) the rate constant is independent of n whereas in reaction (2) it increases nearly linearly with n. When both reactions are possible, the measured n dependence can be explained in terms of contributions from each process. The essentially-free-electron model suggests that the n dependence of the rate constants for Rydberg-atom destruction reflects the energy dependence of the cross sections of the corresponding free electron-molecule collision processes which are $$\eqalignno{&e\sp- + \rm XY \to (XY)\sp{-}\ or\ \to X\sp- + Y&(1\sp\prime)\cr &e\sp- + \rm XY({\it J/}) \to {\it e}\sp- + XY({\it J/}-1)&(2\sp\prime)\cr}$$Analysis of the data therefore provides the behavior of cross sections for these processes at ultra-low electron energies.