The conformational landscape of the medium-size cyclic ketone cycloundecanone has been investigated using chirped-pulse Fourier transform microwave spectroscopy and computational calculations. Nine conformations were observed in the rotational spectrum and identified from the comparison of experimental and theoretical rotational constants as well as the observed and predicted types of rotational transitions. All singly substituted 13 C isotopologues were observed for the most abundant conformer, which allowed the determination of partial substitution and effective structures. The most abundant conformer dominates the rotational spectrum and is almost 40 times more abundant than the least abundant conformer. Conformational preferences are governed by the combination of transannular H···H and eclipsed HCCH interactions.