The formation of thermoreversible gels and aggregates of poly(methyl methacrylate) (PMMA) has been investigated as a function of temperature in two different solvents, i.e., bromobenzene and o-xylene. The molecular structure and the thermal behavior have been determined and correlated. It is again shown that the systems are composed of rigid, interconnected cylinders characterized by a two-population distribution as far as the transverse cylinder radius, r(H), is concerned : a majority of cylinders with r(H) = 1.5 +/- 0.1 nm and a minority of cylinders with r(H) = 3.5-4.O nm. When the gelation sets in, the proportion of the second population is seen to increase substantially at the expense of the first population. These results are interpreted with a fringed-micelle model in which the physical junctions consist of the large-radius population of cylinders. The results and conclusions are discussed in the light of a solvated double-helical conformation.