Ultrafine (“nano”-) particles produced from highly supersaturated vapors or liquids are usually aggregated, often containing thousands of small 'primary’ particles bound together in tenuous structures characterized bymass fractal dimensionsless than 3. Such aggregates have large initial surface area but aremetastablewith respect to more compact configurations. Availablerestructuringmechanisms include surface energy driven coalescence, which, in the case of viscous flow at high gas temperatures, is ultimately able to obliterate all evidence of the original (“primary”) particles. We here exploit the notion that, provided an aggregate is sufficiently large, it can be treated like a spatially non-uniform porous medium, undergoing finite-rate surface energy driven viscous flow sintering leading to final collapse to a single dense sphere. For this purpose, after aDƒ≌s const stage of sintering [associated with a corresponding increase in mean apparent primary particle ('grain’) size], we use an extension of the sintering rate models of Mackenzie and Shuttleworth (1949) and Scherer (1977), treating the material of the restructuring aggregate to be a Newtonian viscous fluid. We predict and report here the time-dependent increase in fractal dimension,Dƒ, and associated decreases in: aggregate outer (maximum) radius, mobility radius, and changes in accessible surface area with dimension-less time [real time in multiples of the characteristic sintering time, μ (R1)t=0/σ cr, where u is the material's viscosity (Rl)t=0is the effective initial grain radius and a the material surface tension]. In these units, we find that thetotal required coalescence timedoes not increase with N as sensitively asN1/3an important observation for processes involving very large aggregates. With validation and the indicated extensions, our pseudo-continuum methods are efficient enough to be used for estimating the morphological- and transport property-evolution of entirepopulationsof restructuring aggregates, perhaps characterized by some non-separable probability density functionpdf(N,Dƒ,R1,) locally, in non-isothermal combustion-synthesis reactors.