We present a set of spectral characteristics distinguishing optically selec
ted active galactic nuclei (AGNs) from starbursts in the mid-IR. In AGN spe
ctra, the Unidentified Infrared Bands (UIBs) are not detected which is prob
ably due to the destruction of their carriers by the UV-X radiation field.
In addition, a non-negligible continuum below 10 mu m commonly associated w
ith emission from very hot dust is exclusively present in AGNs. On the cont
rary, starburst spectra present clearly detected UIBs with a faint underlyi
ng continuum between 5-10 mu m. Based on these features, we build a mid-IR
diagnostic diagram to identify emission induced by the AGN and that associa
ted with the star formation activity. This diagnostic based on the mid-IR c
ontinuum and the UIB intensity allows us to estimate the relative importanc
e of the three components (HII regions, photo-dissociation regions (PDRs),
and AGN) contained in the integrated mid-IR emission of galaxies. Our diagn
ostic diagram can be applied to reveal the presence of very obscured active
nuclei behind large columns of dust.