A new style of diamond anvil cell (DAC) has been designed and built for conducting research in fluids at pressures to 2.5 GPa and temperatures from −190 to 1200 °C. The new DAC has been used for optical microscope observations and synchrotron x-ray diffraction studies. Fringes produced by interference of laser light reflected from top and bottom anvil faces and from top and bottom sample faces provide a very sensitive means of monitoring the volume of sample chamber and for observing volume and refractive index changes in samples that have resulted from transitions and reactions. X-ray diffraction patterns of samples under hydrothermal conditions have been made by the energy dispersive method using synchrotron radiation. The new DAC has individual heaters and individual thermocouples for the upper and lower anvils that can be controlled and can maintain temperatures with an accuracy of ±0.5 °C. Low temperatures are achieved by introducing liquid nitrogen directly into the DAC. The equation of state of H2O and the α-β quartz transition are used to determine pressure with an accuracy of ±1% in the aqueous samples. The new DAC has been used to redetermine five isochores of H2O as well as the dehydration curves of brucite, Mg(OH)2, and muscovite, KAl2(Si3Al)O10(OH)2.