This paper deals with estimation of the diffusion coefficient in an H2O-CO2 system under high pressure and the use of laser-induced fluorescence (LIF) for measuring CO2 dissolution. A liquid CO2 droplet without CO2 clathrate was placed in the flow where the velocity profile is uniform. The diffusion coefficient is estimated by measuring the liquid CO2 droplet-dissolution rate and using the empirical mass transfer coefficient. The measured diffusion coefficient agrees well with the Wilke-Chang equation at pressures of 29.4 and 39.2 MPa. We have used LIF for measurement of the large non-uniform, two-dimensional pH distribution induced by CO2 dissolution. A sheet laser beam was provided by a YAG laser and the two-dimensional LIF intensity was detected by an image-intensified CCD diode-array camera.