When traditional air-to-air cooling is too voluminous, long thermosyphons may offer a way out. For safe operation of heat exchangers equipped with thermosyphons, the limiting heat flux qlim is an important design parameter. Some literature is found to deal with the operation limiting heat flux of closed two-phase thermosyphons. However, R-134a filled thermosyphons with large length-to-diameter (188) are hardly investigated up to now. Extrapolation of existing correlations to predict qlim in this case results in large scatter. The effect of the angle of inclination on qlim has not been considered until now. Dedicated experiments with a single thermosyphon with a large length-to-diameter ratio (188) and filled with R-134a are presented and analyzed. Effects of saturation temperature, filling ratio and angle of inclination, , on the operational limiting heat flux have been investigated. The thermosyphon functions properly if <83 and qlim, is found to increase with increasing . With decreasing saturation temperature, qlim increases. The filling ratio is found not to be crucial if it exceeds 25 %. Correlations are presented to accurately predict the operation limiting heat flux for thermosyphons with a L/d ratio up to 188. Because of the accounting for the above new aspects, these correlations are also relevant for filling refrigerants other than R-134a.