In many commercial processes cellulose is exposed to elevated temperatures in the 100 to 200°C range. Though this range is below the glass transition temperature (Tg) of dry cellulose, in the presence of polar media Tg is depressed, and molecular mobility is sufficient to allow structural reorganization. In the present study, a number of regenerated celluloses, some amorphous and some semicrystalline, were heat treated at elevated temperatures in water under pressure, as well as in other polar media. Two groups of factors were found to influence the type and amount of structural change. The first were the characteristics of the cellulose, that is, the degree of polymerization (DP) and the structure of the cellulose prior to treatment. The other group were the nature of the medium and the time and temperature of the treatment. The amorphous high DP celluloses were found to aggregate as cellulose IV, with the time and temperature determining the extent of ordering rather than resulting in qualitative reorganization. The low DP celluloses, which were of low enough molecular weight to reduce the constraints on molecular mobility imposed by molecular entanglements, had a more diverse pattern of responses. At ambient temperatures the aggregation was into high crystallinity cellulose II; at intermediate temperatures, a variety of forms fitting into the category of cellulose IV were produced; and finally, at high enough temperatures, a cellulose I of very high crystallinity was produced in low yield.