The structure, electron spin resonance, and electrical properties of potassium-loaded zeolite L have been studied asa function of metal loading up to a concentration of nine additional potassium atoms per unit cell. The incorporation of extra potassium into the dehydrated zeolite resulted in an increase in the room-temperature conductivity by a factor of 10 000 at the highest loading level. The strong temperature dependence of the measured electrical conductivities indicates that the conduction process is thermally activated throughout the composition range investigated. In accordance with theoretical predictions by M. J. Kelly, our measurements indicate that any anisotropy in the conductivity must be relatively modest. The role in the conduction mechanism of new K-3(p+) clusters located in the one-dimensional channels is discussed.