University of Gießen. An interactive Loomis-Wood program was applied to identify the c-type transitions of furane and thiophene. The molecular parameters were determined from the observed line positions and the available microwave data by least-squares-fits using Watson's ^-reduced Hamiltonian. Some relations among parameters appearing in different formulations of the Hamiltonian are represented explicitly in connection with the derivation of the unreduced constants. The unreduced constants were derived for both molecules using the planarity relations. We determined the inertial defects to be « 0.05 amuÄ 2 in the ground state and « -0.2 amuÄ 2 in the excited state for both furane and thiophene.