The physical properties of melanosomes have been shown to depend on water content. Herein, the ultraviolet absorption coefficient at λ = 244 nm for intact bovine choroidal melanosomes is determined from photoemission electron microscopy images recorded as a function of vacuum exposure. The dehydration of the melanosome under ultra-high vacuum manifests itself by a decrease in the absorption coefficient to about 60% of its initial value, and a concomitant increase in its image brightness. This change in the absorption of the melanosome is consistent with the influence of solvent polarity on the UV absorption coefficient of model systems for the pigment eumelanin, the predominant UV absorber contained in the choroid melanosomes.