The nitric oxide (NO) binding properties of monolayers of four different protoporphyrins IX adsorbed on aluminum oxide surfaces have been investigated. XPS and AFM results are consistent with the presence of a monolayer of porphyrins, bound to the surface by their carboxylic acid groups and with the porphine ring oriented more or less perpendicular to the surface. Kelvin probe measurements were done in which the samples were exposed to varying NO concentrations. NO binds strongly to all porphyrins, but protoporphyrin IX zinc(II) (Zn-PP) shows the most promising response. The response rate could be increased by adding spacer molecules to the porphyrin layer. Low ppb levels of NO were easily detected in ambient conditions. The NO binding properties of Zn-PP compare favorably to those of other porphyrins which were previously used for NO sensing. This makes Zn-PP a new and promising candidate for this application. © 2012 Elsevier B.V. All rights reserved.