This paper aimed to improve the four-sensor probe methodology for the multi-dimensional two-phase flow measurement. We theoretically derived the interfacial measurement theorem relating the local instantaneous interfacial velocity to local measurable velocities of the multi-sensor probe in the improvement. Based on this theorem, theoretical measurement methods for the local instantaneous interfacial normal direction and the local time-averaged interfacial area concentration (IAC) using the four-sensor probe were presented. An interface-pairing signal-processing scheme was proposed to identify the same interfaces from the sequential signals detected by different sensors. The practical application of the improved IAC methodology to the two-phase flow in a vertical large diameter pipe showed that the four-sensor probes (together with the interface-pairing signal-processing scheme) could effectively measure the local time-averaged IACs with high effective interface percentages not only in the one-dimensional two-phase flow but also in the multi-dimensional two-phase flow. The measurement error analysis indicated that the errors from the bubble deformation and velocity variation due to the sensor piecing were negligible if we only applied the multi-sensor probe to the two-phase flow with the bubbles having much larger size than the sensor diameter. The total error from both the escaped and missing bubbles in the void fraction and IAC measurements was estimated at about 15.75 % in the two-phase flow in a pool. (c) 2005 Elsevier Ltd. All rights reserved.