In this study, an approach to characterize ceramic colloidal suspensions has been developed, based on vitrification of aqueous ceramic suspensions and microstructural characterization using cryogenic-temperature scanning electron microscopy (cryo-SEM), augmented by conventional rheological measurements. The flocculation phenomenon in as-milled and aged Mg-spinel (MgAl2O4) aqueous suspensions was characterized. A microstructure based on hard particle agglomerates separated by long-chain deflocculant molecules was directly observed, and correlated to the rheological properties of the suspension. Several levels of flocculation were detected as a function of suspension preparation conditions. Although suspensions at solid-loading levels appropriate for ceramic processing cannot be characterized using conventional particle size measurement techniques, cryo-SEM can be used to measure characteristic sizes, and to distinguish between agglomerated and flocculated particles, opening a new approach for optimizing solid-loading conditions for slip casting in terms of viscosity and green density.