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Detailed investigations on wall film behavior in port injected SI engines show that a standard UEGO sensor leads to inaccurate results during transients concerning the real air/fuel ratio. Purpose of this paper is to identify the reasons for this behavior and developing a compensation strategy. Fuelling transients due to steps in the injection period at constant engine speed and MAP with modified sensors were carried out and the influence of the protective tube and the dynamic behavior of the measuring cell were found to be the essential parameters. The standard UEGO sensor can be used to determine the air/fuel ratio during transients, if the reasons for inaccurate results presented in this paper are considered.