The effect on oligomerization of increased steric bulk in dimethylindium(III) chalcogenolates (Me2InER′) (E = O, S, Se) has been examined. The facile reaction of Me3In with a series of phenols, thiophenols and selenophenols afforded the compounds [Me2InO(C6H5)]2 (1), [Me2InO(2,6-Me2C6H3)]2 (2), Me2InO(2,4,6-tBu3C6H3) (3), [Me2InS(C6H5)]∞ (4), [Me2InS(2,4,6-tBu3C6H3)]∞ (6), [Me2InSe(C6H5)]2 (7), [Me2InSe(2,4,6-Me3C6H3)]∞ (8) and [Me2InSe(2,4,6-tBu3C6H3)]∞ (9). All compounds have been characterized by elemental analysis, melting point, FT-IR, FT-Raman, solution NMR, and X-ray crystallography. The structures of 1–2 are dimeric via short intermolecular In–O interactions, yielding a symmetric In2O2 unit and a distorted tetrahedral C2O2 bonding environment for indium. Increasing steric bulk in 3 results in the isolation of a monomeric species, exhibiting a distorted trigonal planar C2O bonding environment for indium. In contrast to 1, the thiolate analogue 4 exhibits a polymeric structure viaμ2-SPh groups and a distorted tetrahedral C2S2 bonding environment for indium. Increasing steric bulk resulted in the formation of a chain of weakly coordinated monomers via intermolecular In⋯S interactions in [Me2InS(2,4,6-tBu3C6H2)]∞ (6). Although 7 shows a dimeric species similar to 1, the 2,4,6-trimethyl substituted selenolate analogue 8 exhibits a polymeric structure, while the –Se-2,4,6-tBu3C6H3 analogue (9) showing a similar structure to 6. Comparison to previously reported structures of diorganoindium chalcogenolates demonstrates the importance of the methyl substituents on indium in facilitating the isolation of higher (non-dimeric) oligomers. Theoretical calculations demonstrate the significance of altering the R and R′ groups and E on the degree of oligomerization in [R2InER′]n species.