Abstract An extension to the previous LSCD (lower state combination difference) determination of molecular parameters involving acetylene's ν 5 fundamental and the strongest one quantum hotbands, 2ν 5 ←ν 5 and ν 4 + ν 5 ←ν 4 [ J. Molec. Spectrosc. 146 , 389 (1991)] has been made. A novel iterative numerical diagonalization procedure was employed to fit the vibrational states involved in the seven one quantum hotbands. This method utilizes the Hellmann—Feynman theorem to calculate first derivatives and singular value decomposition (SVD) in its least-square procedure and permits the simultaneous evaluation of the effective dipole moment responsible for the l-type resonance effect upon IR intensities. A set of molecular parameters describing the rotation—vibration levels of the ground state, ν 5 , ν 4 , 2ν 5 and ν 4 + ν 5 for the major isotope and for 13 C 12 CH 2 are reported based upon FT-spectrometric data taken at the McMath Solar Telescope Observatory. The improved spectroscopic parameters retrieved from this investigation will serve as a database for modelling abundances of acetylene in various astrophysical sources.