A study using both archive and new IUE images of time varying mass loss has been carried out for a sample of 18 supergiants in the spectral range B5-B9. Additionally dedicated IUE velocity-time observations of 'discrete absorption components' (DAC) in the UV resonance lines have been made for one of the stars (β Ori), while optical data obtained for this star shows time variability in the H α profile. Ions in which mass loss features occur are unambiguously identified by comparing spectra of the stars at different mass loss episodes. Time variability in the mass loss activity of late B-type supergiants is discussed and the stars are examined for trends with effective temperature. Possible models for the DAC are discussed.