Defect structure and the mobility of point defects in pure metal-deficient cobalt oxide (Co1-yO) and in Co1-yO-Cr(2)O(3) solid solutions have been studied as a function of temperature (1,223-1,573 K) and oxygen pressure (10-10(5)Pa) using microther-mograwimetric techniques. It has been shown that the predominant defects in pure and Cr-doped cobaltous oxide are singly ionized cation vacancies, and 3% at. of dopant is high enough to fix the concentration of predominant defects in such solid solutions on a constant level, being much higher than in pure Co(1-yO. The mobility of point defects in pure Co1-yO has been found to be concentration independent, strongly suggesting that in spite of rather high concentration no interactions and clustering of defects is to be expected. On the other hand, in Cr-doped cobaltous oxide, the defect structure is more complicated, although singly ionized cation vacancies seem to be still predominant defects.