  Spectrophotometry constitutes a unique way to obtain the whole diagnostics
(density, temperature, ionic abundances) of ionized gas nebulae, thus providing
invaluable information about the objects where they reside. If such nebulae are
in the low excitation regime, the diagnostics have to relie on the observation
of the more intense emission lines. We have compiled data for a sample of HII
regions, GEHR and HII galaxies and compared them with results derived from
photoionizacion models whose parameters cover the physical conditions of the
nebulae. Our results confirm a lower uncertainty for the diagnostics when using
empirical methods based on the strong lines of [SII] and [SIII] replacing and
complementing those based on the [OII] and [OIII] lines.
