Geyser boiling in a vertical annular two-phase closed thermosyphon was experimentally studied. The effects of the heat load, condenser temperature, degree of liquid fill and length of the evaporator on the characteristics of the geyser boiling were investigated in detail for both water and ethanol as working fluids. Flow visualization at light heat load clearly illustrates the process of geyser boiling. It also indicates that it occurs more frequently and irregularly at high heat load. The wall temperature measurement dictates that the period of geyser boiling is shorter at a higher heat load, a shorter evaporator length and a smaller liquid fill. Additionally, the heat transfer coefficient is found to increase approximately linearly with the heat load on a logarithmic scale. An empirical equation was proposed to correlate the data for the heat transfer coefficient. A criterion was suggested for the occurrence of geyser boiling based on the present data.