Ab initio HF-SCF molecular geometry optimizations and vibrational mode calculations were performed for furan (I) and the two rotational isomers of 2-furancarboxaldehyde (II). Five basis sets were used to compare the quality of results, from minimal MINI1 to large split 6-31G**. The geometry of I agrees fairly well with accurate experimental determinations in the gas phase, and a full accurate theoretical geometry of each isomer of II is proposed instead of the incomplete experimental values. Calculated vibrational modes of I agreed with experimental results after appropriate scaling of the calculated harmonic modes. Results of a complete vibrational mode analysis for II are reported. The effect of isotopic substitution on vibrational energies is fairly well reproduced for I and II in comparison with reported experimental results after a new computation of the vibrational energies with the same force constant matrix and different atomic masses. Assignments of bands to different isomers of II reopen a discussion about the origin of a carbonyl doublet detected in the liquid phase. Our assignments to C=O stretching bands are based on calculated isotopic displacements, too.