Monte Carlo calculations are reported of calcium- and gadolinium-doped ceria solid solutions, Ce1 - xCaxO2 - x (CDC) and Ce1 - xGdxO2 - x/2 (GDC) as a function of dopant concentration x. Previous work has largely been restricted to the dilute defect limit, made a priori assumptions of the formation of particular clusters, and neglected temperature effects. All these constraints are removed in our study. We examine and compare the formation of Ca and Gd-nanodomains with increasing dopant concentration. The growth of Ca-rich domains in Ce1 - xCaxO2 - x is particularly marked even at low concentrations of calcium. Conductivities of the configurations generated in the Monte Carlo simulations are calculated using molecular dynamics. The Monte Carlo generates the thermodynamically most stable low-energy atomic arrangements and these configurations possess low conductivities relative to those in which the dopants are distributed at random; the nanodomains formed by the dopants reduce oxygen mobility due to the low local concentration of oxygen vacancies and the blocking of pathways for vacancy migration. The calculated conductivity of a Sigma 5(310) grain boundary of Ce1 - xGdxO2 - x/2 with overall composition x = 0.2 is comparable to that of the bulk material despite pronounced segregation to the interfacial region. Overall our results illustrate the importance of kinetic vs. thermodynamic control in synthesis of these systems. (C) 2016 Elsevier B.V. All rights reserved.