Negative ion formation via electron transfer in thermal energy collisions between K(nd) Rydberg atoms and simple polyatomic molecules is studied at low-to-intermediate values of principal quantum number, n (n < 40). At these values of n, the Rydberg electron can no longer be considered simply as a free electron of equivalent energy, because effects associated with the reduced size of the Rydberg atom and the proximity of the atom's charged core become important. We have observed and investigated several novel phenomena, associated with both dissociative and non-dissociative electron transfer. For example, marked n-dependences have been observed in the measured rate constants for free ion production by Rydberg electron attachment. These are due to the rapidly decreasing size of the Rydberg atom which results in atomic opacity, and to the increasing post-attachment electrostatic attraction between the product positive and negative ions. In the case of dissociative Rydberg electron transfer to simple halogenated hydrocarbons, XY, K(nd) + XY → K\sp+ + (XY\sp−)* → K\sp+ + X\sp− + Y where XY is CF\sb3I, CF\sb2Br\sb2, CF\sb3Br, CH\sb2Br\sb2, CCI\sb4, CFCl\sb3, or CHCl\sb3, angular asymmetries were discovered in the velocity distributions of the product negative ions. Analysis of these data provides valuable insight into the translational energy release that accompanies dissociation of the transient intermediate molecular negative ions, and their lifetimes. Measurements of the spatial distributions of the product K\sp+ ions provide additional information on the dissociative attachment process. A new reaction channel was discovered in the case of non-dissociative Rydberg electron transfer to CS\sb2. This channel, which results in the formation of long-lived CS\sb2\sp− ions that undergo rapid electric-field-induced electron detachment in fields of only a few kilovolts per centimeter, is discussed together with a possible theoretical model of this surprising phenomenon.