Abstract: The invention relates to a gas treatment apparatus (1) for treating a gas stream. The gas treatment apparatus (1) comprising addition means (14) for adding a reducing agent (12) to a gas stream passing through the apparatus (1), a guide chamber (8), a mixing conduit (18) and a catalytic treatment chamber (16) containing a catalytic treatment element (20). The apparatus (1) is arranged such that gas must pass through the mixing conduit (18) to pass from the guide chamber (8) to the catalytic treatment chamber (16). The mixing conduit (18) includes an outer wall (22) surrounding a mixing axis (20) within the guide chamber (8). The outer wall (22) including at least one aperture (30) through which a gas can enter the mixing conduit (18) from the guide chamber (8).