The present work reports an in situ XRD analysis on the calcination of natural dolomite under a pure CO₂ atmosphere. The main goal is to provide an insight on how calcination kinetics and the crystal structure of the calcination byproducts are affected by the addition of H₂O to the calciner at a very low concentration. The working conditions are relevant for CO₂ capture from fossil fuel power plants and thermochemical energy storage (TCES) in concentrated solar power (CSP) plants. It is shown that the presence of H₂O in the calciner at the low concentration used in our study (3 vol%) yields a significant acceleration of dolomite decomposition whereas crystallite size of the reaction byproducts is not affected if fine particles are used. On the other hand, decrepitation, which is a well know phenomenon that occurs when relatively large particles are employed, is accompanied by a marked decrease of crystallite size.