When CO2 is injected into a saline aquifer, its flow is controlled by the rock permeability, porosity, chemical composition and fluids, and its state-of-phase which is controlled by the reservoir pressure and temperature conditions. During the injection procedure, CO2 dissolves in the formation water and with time this increasing dissolution enhances trapping mechanism. While monitoring CO2 as a liquid or gas, it becomes equally important to quantify it both in its dissolved and gaseous states. We dissolved CO2 in pure water and passed it through a variable low pressure cell at room temperature. During this time, ultrasonic transmission tests were conducted to monitor the seismic response with CO2 in its dissolved and its gaseous phases. It was found that the signal amplitude was far more sensitive to the amount of dissolved CO2 than velocity, and it was observed that the transmission amplitudes were a function of the density of the dissolved CO2 in brine. These empirical relationships were based on seismic data recorded in a pressure cell containing a CO2:water mixture without a matrix, and a matrix consisting of glass beads saturated with a CO2:water mixture. These data provide reason to believe that while seismic reflectivity may be used successfully for fluid monitoring, the use of transmission seismic such as is found in vertical seismic profiling, may be useful for quantifying fluid in-place, for verification purposes.