The invention relates to an arrangement in connection with an ALD reactor comprising a reaction chamber, the arrangement comprising fittings for feeding a reaction gas to the reaction chamber and for suctioning the reaction gas back, and fittings for feeding a barrier gas. The fittings for feeding and suctioning back the reaction gas and for feeding the barrier gas comprise a middle element having multiple parallel channels which extend through the element, and a first and a second flow-reversing element arranged at ends of the middle element into which the channels open, the flow-reversing elements being arranged to combine the channels in the middle element so as to provide an interchannel flow.