The microwave spectrum of furfural was investigated in the frequency range 7GHz–21GHz and 49GHz–330GHz. The ground and first torsional state of trans-furfural and ground state of cis-furfural were assigned and analyzed. A total of 1720 rotational lines with J up to 100 and K a up to 53 were assigned to the ground state of trans-furfural, 1406 rotational lines with J up to 100 and K a up to 48 were assigned to the first torsional state of trans-furfural and 2103 rotational lines with J up to 90 and K a up to 48 to the ground state of cis-furfural. Accurate sets of centrifugal distortion constants for both conformations have been determined for the first time. The spectra of all 13 C and 18 O singly substituted isotopic species were observed in natural abundance in the 7GHz–21GHz range. Molecular structure co-ordinates, bond lengths and angles of the Kraitchman substitution type (r s ) and pseudo-Kraitchman type (r pKr ) are derived from the isotopic studies.