  An analysis of the OIII and SIII temperatures measurements compiled by
Perez-Montero et al. of emission line objects consisting of HII galaxies, giant
extragalactic HII regions, Galactic HII regions and HII regions from the
Magellanic Clouds, reveals that the OIII temperatures are higher than the
corresponding values from SIII in most objects with gas metallicities in excess
of 0.2 solar. We explore the possibility of temperature inhomogeneities. We
also explored metallicity inhomogeneities by combining two models of widely
different metallicity. We calculate models that consider a non-Mawell-Boltzmann
distributions for the electron energies (kappa parametrization). We also
consider shock heating within the photoionized nebula.
  Varying the various input parameters in the pure photoionization case does
not reproduce the observed nebular temperatures and neither does having local
temperature inhomogeneities. We find that (1) metallicity inhomogeneities of
the nebular gas, (2) shock waves of velocities < 60 km/s propagating in a
photoionized plasma, and (3) an electron energy distribution given by a
kappa-distribution, are successful in reproducing the observed excess in the
[OIII] temperatures. Shock models, however, would require proper 3D
hydrodynamical simulations to become a fully developed alternative while models
with metallicity inhomogeneities appear to fail in metal poor nebulae, since
they result in T_O++(rec)> T_OIII.
