A relatively simple technique has been developed to estimate the interfacial area concentration (ai) of vertically upward gas-liquid slug flow from dual-probe hot-film anemometry measurements. The slug flow is modeled as a series of Taylor bubbles having a regular bullet-like shape separated by liquid slugs containing small spherical gas bubbles. The total interfacial area is the sum of the Taylor bubble surface area and the surface area of the bubbles in the liquid slugs. The ai is estimated from the mean diameter and local void fraction of the bubbles in the liquid slugs, and the length of the Taylor bubbles. These parameters are obtained through selective discrimination of the hot-film signals to separate the Taylor bubbles from the smaller gas bubbles in the liquid slugs. The important non-dimensional parameters that influence the interfacial area concentration are obtained using the synthesis method. A new correlation for ai for vertically-upward slug flow is developed based on the present measurements.