The ongoing interest in very efficient systems for the imitation of cytochrome P-450-dependent monooxygenase reactions, consisting of metalloporphyrin and oxygen donor, prompted us to develop a method to compare the catalytic activity of soluble metalloporphyrins with those which have been immobilised on different silica surfaces. In this article, we describe a method which allows direct comparison between the catalytic activities of soluble and immobilised metalloporphyrins by measuring the NO release of cyclohexanone oxime. We observed that all metalloporphyrins are able to enhance the NO release of cyclohexanone oxime in the presence of iodosobenzene as oxygen donor. In case of cationic metalloporphyrins, no difference in the catalytic activity was determined. In contrast, for perfluorinated metalloporphyrins we ascertained a major loss of activity after immobilisation. These results led us to the assumption that it is not easy to compare quantitative results of NO releases if soluble and immobilised metalloporphyrins were used.