Ice formation in mature, water saturated hardened cement pastes (water/cement ration (W/C) from 0.4 to 0.7) has been measured using the Calvet microcalorimeter. The specimen size was about 11 cm³. The major results are: (a) little ice forms below about -55°C; (b) the rate of ice formation is not a monotone function of temperature, but contains three well defined peaks centered around -8, -23, and -40°C. The relative magnitude of the peaks is a function of the W/C-ratio; (c) melting takes place continuously from -55°C, but by far the major part of the ice melts close to 0°C; (d) the frozen part of the evaporable water varies from 20 percent (W/C = 0.40) to 57 percent (W/C = 0.70); (e) the rate of cooling (in the range 1.7 to 13.3°C/h) has relatively little influence on the amount of ice formed at a given temperature; (f) by far the major part of the ice forms in the pore system.