The formation of crystalline Li4SiO4 and Li2SiO3 from lithium orthosilicate glasses has been studied by means of in situ high-temperature X-ray diffraction. The first phase that crystallizes from the glass could not be identified, but was followed by the transformation to crystalline orthosilicate and metasilicate. Orthosilicate formation was tracked at temperatures between 600 degrees and 650 degrees C, whereas at higher temperatures, the formation of a small amount of crystalline metasilicate occurs. The kinetics of the initial phase of both crystallization processes are described by the Avrami-Erofeev equation, resulting in activation energies of 90 kJ/mol for the formation of the unidentified phase, and 68 kJ/mol for the formation of Li4SiO4. The rate constant for the crystallization of the unidentified phase is 0.014 s(-1) at 510 degrees C, equaling that of the orthosilicate formation at 630 degrees C. After isothermal heat treatment for 100-800 s, depending on temperature, 80%-95% of the sample is crystallized and further crystallization is controlled by diffusion in both cases.