[Ti(acac)(2)((OPr)-Pr-i)(2)] reacts with tert-butylphosphonic acid to yield a series of titanium organophosphonates such as tetranuclear [Ti-4(acac)(4)(mu-O)(2)(mu-(BuPO3)-Bu-t)(2)(mu-(BuPO3H)-Bu-t)(4)]center dot 2CH(3)CN (1), pentanuclear [Ti-5(acac)(5)(mu-O)(2)((OPr)-Pr-i)-(mu-(BuPO3)-Bu-t)(4)(mu-(BuPO3H)-Bu-t)(2)] (2), hexanuclear [Ti-6(acac)(6)(mu-O)(2)((OPr)-Pr-i)(2)(mu-(BuPO3)-Bu-t)(6)] (3), or [Ti-6(acac)(6)(mu-O)(3)((OPr)-Pr-i)(mu-(BuPO3)-Bu-t)(5)(mu-(BuPO3H)-Bu-t)]center dot 2CH(3)CN (4). The isolation of each of these products in pure form depends on the molar ratio of the reactants or the solvent medium. Among these, 3 is obtained as the only product when the reaction is conducted in CH2Cl2. The structural analysis reveals that a simple cluster growth route relates the clusters 1-4 to each other and that a reactive cyclic single-4-ring titanophosphonate [Ti(acac)((OPr)-Pr-i)(2)((BuPO3H)-Bu-t)](2) is the fundamental building block. While the tetranuclear 1 has structural resemblance to the D4R building block of zeolites, the hexanuclear clusters 3 and 4 have the shape of zeolitic D6R building blocks. The presence of adventitious water in the phosphonic acid (arising from small quantities of hydrogen-bonded water) results in the formation of mu-O2- bridges across an adjacent pair of titanium centers in clusters 1-4. To further verify the stability of the hexanuclear cluster over other structural forms, the reaction of (BuPO3H2)-Bu-t was performed with [Ti(acac)(2)(O)], instead of Ti(acac)(2)((OPr)-Pr-i)(2), in CH3CN to yield [Ti-6(acac)(6)(mu-O)(4)(mu-(BuPO3)-Bu-t)(4)(mu-(BuPO3H)-Bu-t)(2)]center dot 2CH(3)CN (5). Compound 5 exhibits a core structure similar to those of 3 and 4 with small variations in the intracluster Ti-O-Ti linkage. Compound 3 is an efficient and selective catalyst for olefin epoxidation under both homogeneous and heterogeneous conditions.