Reverse bias breakdown behaviour of high quality 4H silicon carbide p-n diodes was investigated, using optical and electrical measurement techniques. Most of the sample diodes suffered from early breakdown phenomena in the form of microplasmas at about 80% of the calculated parallel plane breakdown voltage for the diodes, as evident from measured I-V curves. A group of these microplasmas could be correlated to micropipes, identified by optical microscopy, while a large number of microplasmas were caused by other defects and inhomogenities in the space charge region under reverse bias. The same spots that revealed early breakdown phenomena under reverse bias also showed a different electroluminescence (EL) behaviour under low forward current densities compared to those areas with a homogeneous breakdown behaviour. However, even diodes containing one or more micropipes in the region of the junction showed good rectifying behaviour up to two third of the parallel plane breakdown voltage, where the turn-on of a microplasma was observed.