Wave-plate mist eliminators are among the most effective devices to separate liquid from the gas. Liquid separation efficiency in these devices is largely dependent on the gas superficial velocity. However, the upper limit of gas throughput is restricted by the re-entrainment of the deposited liquid from the separator surfaces into the gas stream. There is some knowledge available of re-entrainment phenomena in vertical wave-plate separators. This paper presents experiments that have enabled the mechanisms of entrainment to be identified and the boundaries for air and liquid rates at which entrainment starts for horizontal wave-plate separators to be characterised. It is supported by modelling using an extension of earlier work.