The effect of co-doping yttria-stabilised zirconia with calcia and scandia has been investigated. Changes in the yttrium ion local environment have been monitored using solid-state magic angle sample spinning 89 Y nuclear magnetic resonance. The effect on the low-temperature (below 320°C) bulk ionic conductivity has been observed using AC impedance spectroscopy. It was found that the number of oxygen vacancies in the nearest-neighbour sites to yttrium ions decreased on co-doping with scandia, correlating with an increase in conductivity, but increased on co-doping with calcia, correlating with a decrease in conductivity. This behaviour can be explained by proposing the trapping of oxygen vacancies in the nearest-neighbour yttrium ion sites so that they no longer contribute to the conduction mechanism.