The effect of cyclone length on separation efficiency and pressure drop has been investigated experimentally and theoretically by varying the length of the cylindrical segment of a cylinder-on-cone cyclone. Experimental results based on cyclone lengths from 2.65 to 6.15 cyclone diameters showed a marked improvement in cyclone performance with increasing length up to 5.5 cyclone diameters; beyond this length the separation efficiency was dramatically reduced. Experimental data agreed well with the predictions of a range of models and CFD simulations. This helps to assess the benefit of prolonging a given cyclone. The physical mechanisms behind the observed trends are elucidated. The dramatic fall in separation performance for the longest length was caused by the " natural turning " phenomenon.