The influence of potassium comodification on the structure-property relationships of base (Pb1-3/2x-Lax)(ZryTi1-y)O 3 [(PLZT) 100x/100y/100(1-y)] ceramics has been investigated. Studies were performed by dielectric response, electrically induced polarization and strain methods, and transmission electron microscopy. For 4/65/35, potassium modification resulted in a dramatic decrease (∼10 times) in the magnitude of the induced strain, the development of a pinned polarization, increase in dielectric loss, slight decrease in Tmax, and a dramatic reduction in domain size and change in domain morphology. For 7/65/35 with increasing potassium concentration, only minor changes in the strain-electrical field response was observed and a crossover between nanodomains and normal micron-sized domains occurred. For 12/65/35, significant increases in the magnitude of the induced strain were found, and a significant increase in the nanodomain size occurred. The influence of electrical field on the domain and strain responses of potassium modified PLZT were also performed. These results are discussed in terms of the influence of impurity/defect pinning sites and their distributions on domain stability and polarization switching.