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 by mass fractal dimensions less than 3. Such aggregates have large initial surface area but are metastable with respect to more compact configurations. Available restructuring mechanisms 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 a D-f congruent to 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, D-f, and associated decreases in : aggregate outer (maximum) radius, mobility radius, and changes in accessible surface area with dimensionless time [real time in multiples of the characteristic sintering time, mu (R-1)(t=0)/sigma, where mu is the material＇s viscosity (R-1)(t=0)is the effective initial grain radius and sigma the material surface tension]. In these units, we find that the total required coalescence time does not increase with N as sensitively as N-1/3... an 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 entire populations of restructuring aggregates. perhaps characterized by some nonseparable probability density function pdf(N,D-f,R-1), locally, in non-isothermal combustion-synthesis reactors.