(Mg,Ni)O solid solutions were synthesized by calcination of MgO and NiO mixtures (MgO:NiO = 65:35 in mol) at 1500°C for 2 h in air. When the samples were firstly reduced at 1000°C in a gas mixture of 4 vol% hydrogen and nitrogen, a volume expansion was observed. The results of powder X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDX) analyses suggested that the expansion after reduction was possibly caused by the creation of space between (Mg,Ni)O grains by segregation of nickel particles on (Mg,Ni)O grains. When the reduced samples were re-oxidized at 1000°C in air and then reduced again at 1000°C in a gas mixture of 4 vol% hydrogen and nitrogen, the samples revealed an accelerated expansion, which was much larger than that of the samples reduced once. The morphological characteristics of (Mg,Ni)O grains and segregated nickel particles of the samples those were reduced twice differed to those of the samples reduced once: however, the phases of the all samples were similar, as verified by XRD and SEM analyses. The accelerated expansion after re-oxidation and reduction might be related to changes in the microstructure of the (Mg,Ni)O grains with the nickel particles segregated.