We studied the interaction of a highly ordered array of Co-tetraphenylporphyrin (CoTPP) with NO on Ag(111) by in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. Upon NO exposure, the initially quadratically ordered CoTPP layer reorganizes, showing a wealth of highly ordered NO+CoTPP coadsorbate phases with increasing size of the unit cell, interpreted as due to attractive lateral dipole-dipole interactions between the two species. The findings not only suggest a novel approach to control the arrangement of adsorbed porphyrins in particular but also should generally be considered in the production of functional layers from large organic molecules under ambient conditions or after exposure to small electronegative molecules.