The breakthrough capillary pressure is an important macroscopic property of porous media that is used to predict permeability and to correlate capillary pressure curves. It is normally determined using the porous plate method, where the non-wetting phase is introduced into the medium in increasing steps of pressure until it establishes a continuous pathway through the sample. In mercury porosimetry, breakthrough capillary pressure is identified as the pressure corresponding to about 10-20% non-wetting phase saturation or the point of inflection in the drainage capillary pressure curve. A new method for measuring the breakthrough capillary pressure has been developed, involving a constant rate injection process as opposed to constant, injection pressure. The scope of this paper is to report the accuracy and reliability of the new technique in measuring the break through capillary pressure by reporting test results on transparent micromodels and Berea sandstone core samples. Air or mercury was used as the non-wetting phase while water (or brine, in the case of core samples) was the wetting phase. Constant rate injection was provided by means of a syringe pump, using water to displace a slug of either air or mercury in a tube connected to the porous medium at rates of about 1.8 x 10(-2) cm(3)/min. Pressure was measured at the sample inlet using a pressure transducer and data were recorded using a data acquisition system. The injection pressure vs. time plot reveals the highest pressure established in, such a test, which is identified as the breakthrough capillary pressure. It was concluded that the new test enables the determination of the breakthrough capillary pressure in micromodels with excellent accuracy. The new test was also validated using a sandstone core sample.