We have compared the spectra of six Type Ia supernovae, at ages between 4 and 404 days after maximum light, to see whether all show similar evolutionary behaviour. We find that the spectral lines between 6800 Å and 8000 Å follow a smooth evolutionary trend, but that lines redward of 8000 Å show significant differences. SN 1987I and SN 19880 show extra emission over a broader wavelength region, possibly due to OI 8446 Å emission which would indicate higher envelope densities for these objects. Our results are consistent with the trend of expansion velocity with parent galaxy type reported by Branch and van den Bergh (1993). Variations within the class of Ia SNe may affect their use as standard candles.