A horizontal-flow electrostatic filter for gas passing from a boiler to a chimney and having separate flow chambers lying horizontally side-by-side has inlet and outlet ducting arranged such that flue gases are divided to flow in opposite directions through the respective chambers, both directions of flow being at right angles to the direction of the shortest distance between the boiler house and the chimney. The flue gas output of the boiler is divided equally between the chambers. The size and hence capacity of the filter may be greater, in this disposition, for a given said distance than if a prior art parallel co-current flow arrangement (shown in FIG. 1 of the drawings) were used. Capacity for a given available floor space is further assisted by providing a plurality of arrays of collection and discharge electrodes separately electrically supplied and controlled.