The objective evaluation of the performance of pixel level fusion methods is addressed. For this purpose a global measure based on information theory is proposed. The measure employs mutual and conditional mutual information to assess and represent the amount of information transferred from the source images to the final fused gray-scale image. Accordingly, the common information contained in the source images is considered only once in the performance evaluation procedure. The experimental results clarify the applicability of the proposed measure in comparing different fusion methods or in optimizing the parameters of a specific algorithm.