A general criterion for the existence of phase separation in driven density-conserving one-dimensional systems is proposed. It is suggested that phase separation is related to the size dependence of the steadystate currents of domains in the system. A quantitative criterion for the existence of phase separation is conjectured using a correspondence made between driven diffusive models and zero-range processes. The criterion is verified in all cases where analytical results are available, and predictions for other models are provided. DOI: 10.1103/PhysRevLett.89.035702 PACS numbers: 64.75. +g, 05.20. -y The existence of phase separation and spontaneous symmetry breaking in low-dimensional systems far from thermal equilibrium has been a subject of recent interest [1,2]. While it is well known that these phenomena do not take place in one dimension in thermal equilibrium, several models of driven one-dimensional systems with local dynamics have recently been demonstrated to exhibit both [3 -5]. Whether or not a given model exhibits phase separation is in many cases not a simple question to answer, and it may depend on numerical evidence which could be rather subtle.For example, in a recent three-species model introduced by Arndt et al. [4] (AHR), it has been suggested that one should expect two distinct phase separated states: one in which the three species are fully separated from each other (related to the phase separation observed by Evans et al.[3] in a related model) and the other is a more subtle mixed state whose existence is supported by extensive numerical simulations of systems of finite length and by a meanfield treatment. Subsequently, an analytical analysis of the model has shown that the mixed state is in fact disordered and that in order to see this in simulations one has to study extremely long systems (of the order of 10 70 ), far beyond existing numerical capabilities [6].In another example introduced by Korniss et al. a twolane extension of a three-species driven system was studied [7]. It has been suggested that while for this model the one-lane system does not exhibit phase separation [8], this phenomenon does exist in the two-lane model. The studies rely on numerical simulations of systems of length up to 10 4 . This result is rather surprising and not well understood. It may very well be the case that as for the AHR model, the two-lane model does not actually exhibit phase separation in the thermodynamic limit and that this could be seen only by studying extremely long systems. It would thus be of great importance to find other criteria, which could distinguish between models supporting phase separation from those which do not.In this Letter we introduce a simple general criterion for the existence of phase separation in density-conserving one-dimensional driven systems. Phase separation is usually accompanied by a coarsening process in which small domains of, say, the high density phase coalesce, eventually leading to macroscopic phase separation. This process takes place as domains exc...