The use of the Baldwin et al. (1981) or Veilleux & Osterbrock (1987) diagnostic diagrams allows the unambiguous classification of the nuclear emission line regions of most galaxies into one of three categories: nuclear HII regions or starbursts, Seyfert 2 galaxies and Liners. However, a small fraction of them have a "transition" spectrum. We present spectral observations of 15 "transition" objects at high-dispersion (66 Å/mm around the H
,
[NII] 
6548,6584 and/or
H
,
[OIII]
4959,5007 emission lines.
We show that most of these spectra are composite, due to the
simultaneous presence on the slit of a Seyfert nucleus and a HII region.
Seyfert 2s and Liners seem to occupy relatively small and distinct volumes
in the three-dimensional space 5007/
H,
6584/H,
6300/H.