Seeds of Adenanthera pavonina and Leucaena leucocephala, belonging to the Leguminosae family, Mimosoideae subfamily, were treated with boiling water for enzyme inactivation and the isolated endosperms submitted to successive aqueous (4ºC, 25ºC and 60ºC) and aqueous alkaline extractions (2M NaOH and 4M NaOH), giving rise to galactomannans, which are typical reserve polysaccharides from leguminous seeds. Galactomannans consist of a (1®4)-linked, b-D-mannan backbone which is substituted at O-6 by a-D-galactopyranosyl units, ranging from 5% to 100% substitution. The galactomannans from A. pavonina and L. leucocephala had a Man:Gal ratio of 1.6:1 for both, independent on the extraction conditions. The structure of these polysaccharides were confirmed by methylation analysis and ¹³C-NMR. After two additional extractions with 4M NaOH at 25ºC, the major monosaccharide found in the final residue of the Mimosoideae species was glucose, indicating the presence of cellulose. The same procedure was used for seeds of Cassia fastuosa and Delonix regia (Leguminosae family, Caesalpinioideae subfamily) and the Man:Gal ratio of the galactomannans obtained ranged from 2.5:1 to 10.0:1 for C. fastuosa and 3.0:1 to 6.8:1 for D. regia, from a family of galactomannans. The final residue obtained after the alkaline extractions was characterized by methylation and ¹³C-NMR as a linear pure mannan, usually found in the endosperm of Palmae species. The endosperm from Leuguminosae-Caesalpinioideae, Schizolobium amazonicum (PETKOWICZ et al., 1998; PETKOWICZ, et al., 2001) and Schizolobium parahybae, also contains a family of galactomannans and a pure mannan. Considering the evolutionary pattern for Leguminosae, in which Mimosoideae is derived from Caesalpinioideae, these results indicate that during evolution the synthesis of a galactomannan family was probably substituted by a single polysaccharide with an increase of the galactose proportion and the substitution of a mannan by cellulose. Supported by CNPq and PRONEX.