PEEM (photoelectron emission microscopy) was employed to study the influence of a noisy reactant gas flux composition on the spatio-temporal pattern development in the CO oxidation reaction on stepped Ir(9 7 7) surfaces. Anisotropic diffusion of CO parallel and normal to the steps causes elliptic shapes of large oxygen islands at small noise amplitudes. The long axes of the ellipses are aligned along the steps. At increased noise amplitudes the density of oxygen island ellipses becomes larger while preserving their alignment. 2D modeling of the phenomena based on reaction-diffusion differential equations reproduces the experimental findings quite nicely. (c) 2006 Elsevier B.V. All rights reserved.