Development of successively higher field nmr spectrometers has facilitated the study of increasingly more complex molecules, although smaller molecules such as phenanthro[3,4-blthiophene still offer very substan­tial assignment problems because of the highly congested nature of their 1H- and 13C-nmr spectra. Assign­ments of such spectra, if they are to be unequivocal, frequently require the utilization of two-dimensional nmr spectroscopic techniques. Total assignments of the 1H- and 13C-nrnr spectra of phenanthro[3,4-b]thio­phene are reported. Assignments were based on a conventional high resolution 500 MHz 1H-nmr spectrum, autocorrelated two-dimensional 1H-nmr spectra (COSY), two-dimensional 1H-13C chemical shift correlation spectra and a modified version of autocorrelated 13C-13C double quantum coherence two-dimensional nmr spectroscopy. From NOE measurements, a separation of 1.99 Å between H1 and HI 1 was computed, suggesting that phenanthro[3,4-b]thiophene has a pronounced helical conformation in solution.