This paper presents an experimental study of an air cooled vertical two-phase tubular copper thermosyphon with water as the working fluid. Its internal diameter is 10mm and total length is 400mm with an evaporator fill ratio of 50%-55%. Heat was supplied in increments of 25W from 50W to 200W. The total thermal resistance of the thermosyphon was 0.258K/W at 200W. The operational regime was thought to be geyser at lower heat loads (below 175W) and churn at higher heat loads. The evaporator was separated into distinct pool and film regions for heat loads less than 175W, however above this the churn regime homgenized the temperature distribution. The Rohsenow pool boiling model in conjunction with El-Genk and Saber film evaporation/boiling model gave a reasonable estimate of the evaporator temperature gradient. However these models break down once the churn regime occurs.