  We recapitulate a thermodynamically consistent excluded volume hadron gas
model and examine its differences with other ``thermal models'' used in the
literature. Preliminary experimental data for particle number ratios in the
collisions of Au+Au at the BNL AGS (11$A$ GeV/$c$) and Pb+Pb at the CERN SPS
(160$A$ GeV/$c$) are analyzed. For equal values of the hadron hard-core
parameters the excluded volume model gives essentially the ideal gas
predictions for the particle number ratios, which is similar to other thermal
models. We observe, however, the systematic excess of experimental pion
abundances compared to the ideal gas results. This effect can be explained in
our model by a smaller pion hard-core volume compared to those of other
hadrons. The absolute values for particle number and energy densities at the
chemical freezeout are predicted with a simultaneous fit to all these AGS and
SPS particle number ratios.
