Rydberg electron transfer spectroscopy (RET) has been used to determine the dipole-bound electron affinity of the indole molecule, and the value of 3 meV was obtained. RET has also been employed to study [indole-(water)(N)](-) cluster anions and the results have been interpreted with the help of ab initio calculations. It has been shown that for N=1 and 2 only dipole-bound anions are formed and that the electron attachment induces large amplitude motions in these systems. [Indole-(water)(N)](-) anions with N=3 and 4 have not been observed. This finding for N=3 is consistent with a low theoretically predicted dipole moment of the neutral indole-(water)(3) complex, which is insufficient for the formation of a stable dipole-bound anion. Above N=5, RET experiments showed formation of valence [indole-(water)(N)](-) anions. From the observed size threshold for the formation of these anions, the negative value of the valence electron affinity of indole equal to -1.03 +/- 0.05 meV was deduced. (C) 2000 American Institute of Physics. [S0021-9606(00)00408-6].