The concept and occurrence of isoreticular (same topology) series of metal organic frameworks (MOFs) is reviewed. We describe the preparation, characterization, and crystal structures of three new MOFs that are isoreticular expansions of known materials with the tbo (Cu-3(4,4',4 ''-(benzene-1,3,5-triyl-tris(benzene-4,1-diy1))tribenzoate)(2), MO F-399) and pto topologies (Cu-3(4,4',4 ''-(benzene-1,3,5-triyltribenzoate)(2), MOF-143; Cu-3(4,4',4 ''-(triazine-2,4,6-triyl-tris-(benzene-4,1-diyl))tribenzoate)(2), MOF-388). One of these materials (MOF-399) has a unit cell volume 17 times larger than that of the first reported material isoreticular to it, and has the highest porosity (94%) and lowest density (0.126 g cm(-3)) of any MOFs reported to date.