Recent studies have suggested that tube diameter is a relevant parameter which should be included when correlating heat transfer data for pool boiling on horizontal tubes. However, there are few data for tubes of diameter less than 8 mm, and the diameter effect for small tubes has not been determined. This paper reports the results of an experimental investigation using tubes of 1-6 mm diameter. Tests have been carried out with water and refrigerant R141b at heat fluxes ranging from 1 to 90% of the critical heat flux. The results indicate that correlations which show an inverse relationship between heat transfer coefficient and diameter should not be applied for tubes of less than 6 mm diameter. Conventional pool boiling correlations derived from data acquired from boiling on wires and flat plates yielded acceptable results when applied to the test data from this study.