The microwave spectrum of methyl isobutyl ketone was recorded using a molecular beam Fourier transform microwave spectrometer and analyzed to determine the rotational constants A = 4.7517(17) GHz, B = 1.496115(79) GHz, C = 1.324364(39) GHz, and centrifugal distortion constants. A-E splittings from 250 MHz up to 1 GHz were observed due to internal rotation of the acetyl methyl group with a potential barrier of 250.3(19) cm(-1). 59 A and 57 E species lines were fitted to experimental accuracy. The experimental rotational constants were compared with results from quantum chemical calculations to validate the molecular structure. (C) 2012 Elsevier B. V. All rights reserved.