The microwave spectrum of isobutylbenzene (2-methyl-1-phenylpropane) reveals the presence of two conformers that are characterized by their microwave spectra and by quantum chemical calculations. The more stable conformer has a gauche configuration of the C(phenyl)-C-1-C-2-H chain coupled with a similar to 80 degrees dihedral angle between the phenyl group and the C(phenyl)-C-1-C-2 plane with C-1 symmetry. The less stable conformer has a plane of symmetry, C-s, with all anti Configuration of the C(phenyl)-C-1-C-2-H chain coupled with a 90 degrees dihedral angle between the phenyl group and the C(phenyl)-C-1-C-2 plane. The rotational constant Values are 3070.9273(4) MHz, 736.01980(6) MHz, and 680.92889(6) MHz for the C-1 species and 2500.780(8) MHz, 885.727/43( 10) MHz, and 770.42036(10) MHz for the C-s species. Quantum chemical calculations are in agreement with these structures and predict a relative energy between those two conformers of 0.4 kcal/mol.