In the last decade diode-array detection has proved to be extremely useful in high performance liquid chromatography in recording UV-visible spectra directly and on-line in the column effluent. In capillary electrophoresis (CE) only fast-scanning detectors with long scan times (up to 2 s) are commercially available. A home-made CE instrument with a diode-array detector (DAD) was built and compared with a commercial fast-scanning detector. It is shown that the fast-scanning detector is advantageous when less than eight wavelengths are monitored. Due to the high data collection rate the DAD is superior if high resolution UV-visible spectra are acquired or if several spectra of a narrow peak (usual peak width in CE below 5 s) are needed for peak purity control. It is shown that DAD detection can help to develop separation methods, to identify peaks according to their UV-visible spectra, and to check peak purity.