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We propose to use ISOCAM to image the circumstellar envrionment of luminous blue supergiants in the CO fundamental band at 4.6 micron. Spectroscopic surveys from the ground have revealed the surprising presence of 2.3 micron CO overtone bands in emission in about a dozen very hot, very luminous, and clearly evolved supergiants. We present three hypotheses that 1) explain the presence of CO as a transient penomenon, associated with certain activity states of these stars, 2) that place the CO in a distant region from the stars where a fast supergiant wind compresses the outlow from a mass-losing progenitor, and 3) that associates CO with dense, self-shielding circumstellar disks. ISOCAM is ideally suited to test these three hypothesis. The CO observations are expected to have fundamental impact on our understanding of massive star formation and massive star evolution. In particular, if the CO emission is extended, we would conclude that it arises in the post-main-sequence mass-loss environments of massive stars. Our results would favor an interacting-winds model for the formation of nebulae around massive stars, similar to the now well accepted formation mechanism of Planetary Nebulae around lower-mass stars. If, on the other hand, CO emission is detected from the stellar positions, we would conclude that it arises in dense, circumstellar disks. These disks could be interpreted as prenatal disks considering the current paradigm for low-mass star formation and the fact that massive stars evolve extremely rapidly. Whatever the results, the proposed CO images will provide innovative new insights into the mass loss process in the upper HR diagram.