Abstract: Authors present an ultrasonic method for measuring the role of dissolved gas in a liquid. The aim of this analyse is to control the gas rate in a low temperature and high pressure flow of liquid hydrogen or liquid oxygen. The interest is to prevent the defective behaviour of carburant loading pumps in rocket engine. A preliminary study is realised on a water column, without average flow, to the basis of which are generated bubbles of air. The ultrasonic method used to detect the gaseous phase appearance consists in recording the amplitude of waves transmitted through the two-phase flow. A technical is proposed to measure average and local gas rate. Acoustic tests have been realised on two types of columns (glass tube with square section and polymetacrylate tube of round section). In each configuration and for several measuring frequencies (134 to 600 kHz), results show a quasi-linear decreasing of the amplitude (in logarithmic coordinate) as the air gas rate increases (0 to 9%).