Ammonia absorption process of ammonia vapor into ammonia water solution has been investigated experimentally, by inserting superheated ammonia vapor into a test cell containing a stagnant pool of ammonia water solution of several ammonia mass fractions, Ci. Before commencing the experiment, the pressure in the test cell corresponds to the equilibrium vapor of the ammonia-water system at room temperature. When the valve is opened, mechanical equilibrium is established quickly and the ammonia vapor diffuses into ammonia solution . The difference between the initial pressure in the vapor cylinder and the initial pressure in the test cell ΔPi is found to have a major influence not only on the absorption rate but also on the estimated interface concentration. The interface concentration Cint of the cases ΔPi = 50 and 100 kPa exhibits a similar tendency, Cint decreases rapidly compared to other initial pressures ΔPi = 150 and 200 kPa. On the other hand, the interface concentration Cint of the cases ΔPi = 250 and 300 kPa are increasing within about 50 sec, then are hardly changing with time. They behave almost in a similar way as of Cint = 0.27 kg/kg. A correlation which gives the total absorbed mass of ammonia as a function of the initial concentration, the initial pressure difference and time is derived. In addition, the absorbed mass at no pressure difference could be estimated from the absorbed mass at initial pressure difference.