The kinetics of the thermal decomposition of a limestone and, simultaneously, of the lime growth have been investigated during isothermal treatments by means of in situ High Temperature X-Ray Powder Diffraction. In particular, four grain sizes of a quarry limestone with differences in terms of mineralogical composition (dolomite content), chemical composition (Fe and Mg amounts), and texture, have been studied. The lime produced during the isothermal treatments was then examined by using Scanning Electron Microscopy. In all samples, limestone decomposition starts with dolomite dissociation, followed by calcite decomposition. Rather than particle size, limestone texture, and dolomite content seem to be the most important factors controlling the starting temperature of calcination and its kinetic behavior.