We present the spectral evolution, from about one to six months after optical maximum, of two type Ic supernovae, 1990U in NGC 7479 and 1991A in IC 2973 (UGC 6872). We analyze and compare the SN 1990U and SN 1991A spectral sequences. We looked for differences, at similar phases, between the more characteristic spectral features and their evolution. From this analysis, we have found that the overall spectral evolution of these two supernovae followed the same trend. The onset of their nebular phases must have occurred not later than three months after optical maximum. However, we have found some differences between these objects as derived from their spectra. We suggest that SN 1991A was a more oxygen rich and helium poor supernova than SN 1990U.