Analysis of postulated loss-of-cooling accidents (LOCA) in CANDU 6  reactors has to consider oxidised state and Zr-2.5%wtNb alloy microstructure. During thermal transients, in oxidized alloy, microstructure changes in oxide layer and material base have been observed by optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS). Samples isothermally oxidized at 700⁰C, with variable thickness oxide layers, were subjected to the various temperature transients up to 1000⁰C, at different controlled heating/cooling rates. The effects of oxide and alloy microstructural changes are correlated with the characteristics of thermal transients.