An improved method is disclosed for fracturing an oil well or gas well with a mixture of liquid carbon dioxide and liquid petroleum gas. The objective is to be able to inject the liquid into the wellbore at a relatively high pumping rate without causing the liquid to boil. Prior to injection, both the liquid CO.sub.2 and the LPG are held in separate supply tanks at a temperature and pressure at which the liquid phase will not boil. The temperature of the LPG is substantially higher than the liquid CO.sub.2. During the pumping operation, part of the liquid CO.sub.2 and all of the LPG is fed through a heat exchanger. In the exchanger the amount of heat transferred from the LPG to the liquid CO.sub.2 is enough to vaporize the liquid. The CO.sub.2 vapor is then circulated back into the CO.sub.2 tank. The recycled vapor thus maintains the liquid-vapor phase in the tank at equilibrium, so that the liquid will not boil at the desired pumping rate.