The invention relates to an arrangement for feeding a treatment substance to an application device used to treat a moving paper or board web, which arrangement includes at least one application chamber (1) extending over the width of the web, to which the treatment substance can be fed. The treatment substance feed devices include at least one elongated feed chamber (3) extending over the width of the application device, to which treatment substance can be fed from the inlet side (8) of the chamber (3), and at least one connection (20) for leading the treatment substance from the feed chamber to the application chamber (1). For feeding the treatment substance between the feed chambers, there is at least one flow route to connect the end (19) of the feed chamber (3) opposite the inlet side (8) and the end of the application device on the side of the inlet side of the first feed chamber (3), through the flow route.