Recent experimental studies of a vibrated granular bed consisting of two 
particle types have shown a transition from a mixed to a segregated state 
as the filling fraction is varied.  A model of the system is introduced 
and investigated numerically.  Many of the experimentally observed 
features are reproduced and it is shown that the mixed state to segregated 
state transition is a robust phenomenon.  The model is used to investigate 
and elucidate the segregation mechanism(s) present in the experiment.