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.