An indirect method of measuring gas holdup in-gas-liquid bubble column reactors has been developed. This technique is based on the analysis of the ultrasonic wave transmitted through the two-phase flow. Gas holdup measurements : have been made on water-nitrogen bubble system at ambient conditions. The data clearly show that the attenuation of the sound is a well-defined function of the gas holdup in the bubble column for homogeneous flow regime only (i.e., the superficial gas velocity is 4 cm/sec or less).