The microwave spectrum for the 1,2-cyclohexanedione (monoenolic)-formic acid dimer was measured for four isotopologues in the 4.5-9 GHz range using a Flygare-Balle type spectrometer. Rotational and distortion constants (A, B, C, D-J, D-JK) were obtained. Measured rotational constants were used in a least squares fit to determine some of the gas phase structural parameters of the dimer. Rotational constants and distortion constants of the parent isotopologue are A = 2415.044(18)MHz, B = 543.6907(2)MHz, C= 451.6663(2) MHz, D-J = 0.0220(13) kHz, and D-JK = 0.119(31) kHz. The experimental hydrogen bond lengths are 1.97 A, somewhat longer than the values calculated using GAUSSIAN 09 with MP216-311++G**. (C) 2014 Elsevier B.V. All rights reserved.