The partition curve is used to quantify the classification in hydrocyclones. It is generally monotonic and asymptotes to a value called the bypass. A ＇fish-hook＇ curve occurs sometimes when partition values lower than the bypass are observed. A number of attempts have been made to develop fish-hook partition curve models. These are reviewed and shown to reduce to a general form. The general equation consists of two competing effects: that of classification, described by a conventional corrected partition curve, and that of dispersion, described by an inverse corrected partition curve that is applied to only the bypass fraction. A turbulence model for two-phase systems that quantifies the relative effects of dispersion and classification is described and shown to be applicable to this system. This allows some physical interpretation of the effect of variables on the observed performance. A series of small diameter hydrocyclone experiments illustrate the use of the model and are used to evaluate the bypass. It is often assumed that the bypass can be estimated accurately from the fraction of water in the feed that reports to the coarse product stream. It was, however, found that the recovery of water to the underflow was significantly lower than either the lowest point of the partition curve or the value of the bypass. Further work is required to conclusively determine the variation in the bypass with operating conditions.