Aqueous solutions that contain volatile (gas) components are one of the mos
t important types of fluid in the Earth's crust. The record that such fluid
s have left in the form of fluid inclusions in minerals provides a wealth o
f insight into the geochemical and petrologic processes in which the fluids
participated. This article reviews the systematics of CO2-H2O fluid inclus
ions as a starting point for interpreting the chemically more complex syste
ms. The phase relations of the binary are described with respect to a quali
tative P-T-X model, and isoplethic-isochoric paths through this model are u
sed to explain the equilibrium and non-equilibrium behaviour of fluid inclu
sions during microthermometric heating and cooling. The P-T-X framework is
then used to discuss the various modes of fluid inclusion entrapment, and h
ow the resulting assemblage textures can be used to interpret the P-T condi
tions, phase states, and evolution paths of the parent solutions. Finally,
quantitative methods are reviewed by which bulk molar volume and compositio
n of CO2-H2O fluid inclusions can be determined from microthermometric obse
rvations of phase transitions. (C) 2001 Elsevier Science B.V. All rights re
served.