Particle size distributions are sometimes conveniently described in terms of their integral moments. It is known that the details of a size distribution cannot always be recovered from its moments, but it has also been shown that an aerosol's optical properties can be insensitive to these details. This paper explores the limits of the correspondence between optics and moments, exhibiting size distributions with identical moments but dissimilar optical properties, and dissimilar moments but similar optical properties. The examples involve multimodal size distributions of the type observed in continental ambient air. ¿ 1998 American Geophysical Union