Investigation into the early hydration of Portland cement was performed by in situ X-ray diffraction (XRD). Technical white cement was used for the XRD analysis on a D5000 diffractometer (Siemens). All diffraction patterns of the in situ measurement which were recorded up to 22 h of hydration at defined temperatures were analyzed by Rietveld refinement. The resulting phase composition was transformed with respect to free water and C-S-H leading to the total composition of the cement paste. The hydration reactions can be observed by dissolution of clinker phases as well as by the formation of the hydrate phases ettringite and portlandite. With increasing temperatures the reactions proceed faster. The formation of ettringite is directly influenced by the rate of dissolution of anhydrite and tricalcium aluminate (C3A). The beginning of the main period of hydration is marked by the start of portlandite formation. The experiments point out that a quantitative phase analysis of the cement hydration is feasible with standard laboratory diffractometers.