After an experimental study of refrigerants HFC-134a, HCFC-123, HFC- 404A and HFC-407C, and considerable modifications to the test stand, LENI has undertaken a new experimental program and has assembled a large new database for evaporation of ammonia and two ammonia-oil mixtures (1% and 3% wt. oil) inside horizontal tubes with smooth and enhanced surfaces. This work extends the current database to include a refrigerant in which there is a marked renewal of interest. The comparison of the ammonia results with the flow boiling heat transfer model of Kattan et al.(1997) will allow the model to be validated to a much broader range of application The experimental tests on smooth tubes without oil covered a wide range of mass velocity (20-140 kg/(m2s)) and heat flux (5-70 kW/m2). However some additional tests on evaporation for a wide range of mass velocities at constant vapor quality are still required as part of the database in order to investigate flow pattern transitions and to develop a new version of the Kattan et al.(1997) model with a wider field of application to fully stratified and stratified-wavy flow patterns. The predicitons of the heat transfer coefficient for the annular and intermittent flow patterns by the Kattan et al.(1997) model produced very convincing results (average deviation of -11.9% and standard deviation of 6.33%) calculated over 172 measurement points), in spite of the fact that the thermophysical characteristics of ammonia differ considerably from those of traditional refrigerants. The effect of 1% and 3% oil on plain and microfin tube heat transfer at 50 and 80 kg/(m2s) was similar to that found for annular flows with previous refrigerants, i.e. little influence at low to intermediate vapor qualities but a substantial fall off in performance at vapor qualities greater that 70%. At the lowest mass velocity of 20 kg/(m2s), the 3% oil tended to increase the plain tube performance while for the microfin tube the oil holdup in the test section was so severe for both 1% and 3% oil that the heat transfer coefficients fell towards zero at vapor qualities greater than 40%. Thus, very low mass velocities when using microfins should be avoided since a little oil will invariably be present in an operating unit. A study on two-phase pressure drops in tests without oil and with nominal oil concentrations of 1% and 3% showed that they remain the same or decrease in the presence of oil. The decrease is surprising result and were confirmed by several independent means of measurement, and merit more future in- depth study.