The hydration kinetics of a white cement and batches of both Class G and H oil-well cements were examined between 0 and 60 MPa, at ≤80°C, using in situ synchrotron X-ray diffraction. This gives a continuous measure of the C3S (Ca3SiO5), CH (Ca(OH)2), C4AF (Ca2FeAlO5), ettringite, and other phases in the hydrating slurries. Slurries prepared from single-phase C3S; synthetic C4AF, and gypsum; and white cement, synthetic C4AF and gypsum were also examined. An increasing pressure enhanced the rate of hydration for all slurries. Analysis of the data, using a kinetic model, provided rate constants that were used to obtain activation volumes for C3S hydration. For all the cement and C3S slurries studied, similar activation volumes were obtained (average ΔV‡∼−35 cm3/mol), indicating that the presence of cement phases other than C3S has a modest influence on the pressure dependence of C3S hydration. An alternative analysis, using the time at which 90% of the initial C3S remained, gave similar activation volumes. Pressure accelerated the formation of ettringite from synthetic C4AF in the presence of gypsum. However, in slurries containing cement, the pressure dependence of C3S hydration plays a major role in determining the pressure dependence of ettringite formation.