Fluid containing free gas and in a pipeline or container at a refinery or at any of various petroleum producing operations is bombarded with neutrons and high energy gamma rays resulting from capture of thermal neutrons are detected. The spectra of the detected gamma rays are then analyzed to determine the concentration of the elements chlorine and hydrogen. The counting rate for thermal neutron gamma rays for hydrogen permits a measure of the hydrogen index (HI) of the fluid to be obtained. The hydrogen index and the relative concentrations of chlorine and hydrogen are used to obtain an indication of the presence and concentration of chlorine or salt water in the fluid.