There are three isomers of toluidine, which are organic compounds discovered and named by James Sheridan Muspratt and August Wilhelm von Hofmann in 1845. These isomers are o-toluidine, m-toluidine, and p-toluidine, with the prefixed letter abbreviating, respectively, ortho; meta; and para. All three are aryl amines whose chemical structures are similar to aniline except that a methyl group is substituted onto the benzene ring. The difference between these three isomers is the position where the methyl group (–CH<sub>3</sub>) is bonded to the ring relative to the amino functional group (–NH<sub>2</sub>); see illustration of the chemical structures below.
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{| class="wikitable" border="1" style="text-align:center; background:#ffffff;"
! colspan="4" style="background:#ccccff;" | Toluidine isomers
|-
| Methyl position
| ortho
| meta
| para
|-
| Common name
| o-toluidine
| m-toluidine
| p-toluidine
|-
| Other names
| o-methylaniline
| m-methylaniline
| p-methylaniline
|-
| Chemical name
| 2-methylaniline
| 3-methylaniline
| 4-methylaniline
|-
| Chemical formula
| colspan="3" | C<sub>7</sub>H<sub>9</sub>N
|-
| Molecular mass
| colspan="3" | 107.17 g/mol
|-
| Glass transition temperature
| 189 K
| 187 K
| Glass not formed