Pressure-temperature coordinates of critical points for binary mixtures of CO2 and five co-solvents are estimated using a peak-shape-sensitive flow-injection procedure. CO2 is pumped continuously under pressure control into a thermostated capillary tube which is restricted at its outlet and interfaced to a flame ionization detector. The co-solvent is injected into the capillary tube inlet at ambient temperature, where the CO2 is a liquid at the test pressures and transported to the thermostated section of the capillary tube. The shape of the co-solvent peak as it exits the capillary tube and enters the detector is dependent on mass transport rates and phase behavior occurring earlier in the system.