Motivated by controversies in the literature on the microscopic and mesoscopic structure of some aqueous solutions, we have performed static and dynamic light-scattering experiments in aqueous solutions of 3-methylpyridine (3MP) and tertiary butyl alcohol (TBA). In addition to the microscopic concentration fluctuations we have found the presence of reproducible mesoscopic inhomogeneities, which become especially pronounced below room temperature. We find that the observed inhomogeneities are near-spherical Brownian aggregates of a size from a hundred to a few hundred nm. We speculate that these aggregates are long-lived metastable clathrate-like precursors triggered by minute traces of impurities in these solutions.