Experimental studies on natural convection boiling of water in an internally heated narrow vertical annulus, with the liquid circulating through a cold leg forming a closed-loop thermo-siphon, have been carried out. The radius and aspect ratios of the annulus are 1.184 and 352, respectively. The experimental data, which consist of wall and liquid temperatures, liquid and vapor flow rates, and differential pressure across the test section, are recorded on a data logging system. The experiments have been performed for a constant heat flux of 15-35 kW/m(2) from the startup period until the steady state to study the transient behavior of the system. The boiling and non-boiling zones in the annulus have been identified and presented graphically through the liquid and wall temperatures for the steady state. They have been also verified through the visual photographs of the flow patterns in the annulus. The flow is found to be oscillatory in nature with no particular trend. Although the experimental data seems to be scattered, but when analyzed for a short duration, they are found well within the +/- 3 sigma (three sigma). This confirms the quasi-steady-state condition of the system. The steady-state values of Reynolds number and liquid circulation rate come out to be 133.1-453.5 and 7.0-23.87 g/s, respectively, while the Nusselt number and heat transfer coefficient are 7.98-13.57 and 1433.57-2435.35 W/m(2)K, respectively. Mathematical correlations for liquid mass flow rates, heat transfer coefficient, Reynolds number, and Nusselt number have been developed and compared with the existing correlations, which are in good agreement.