Abstract
We analyse the distribution of 482 planetary nebulae (PNe) within the Hα/Hβ - Hγ/Hβ and Hδ/Hγ - Hγ/Hβ planes. Whilst most sources appear to possess case B line ratios, and normal reddening decrements, certain of them also appear to be affected by Balmer self-absorption. We are able to identify 9 possible cases, and 11 probable examples where such absorption is important. Approximately half of these nebulae are compact, and probably quite young. Most previous identifications of self-absorption are shown to be open to doubt, and there is no overlap at all between our present candidates, and those of previous analyses.
