  We calculate thermal photon and neutral pion spectra in ultrarelativistic
heavy-ion collisions in the framework of three-fluid hydrodynamics. Both
spectra are quite sensitive to the equation of state used. In particular,
within our model, recent data for $S+Au$ at $200$ AGeV can only be understood
if a scenario with a phase transition (possibly to a quark-gluon plasma) is
assumed. Results for $Au+Au$ at $11$ AGeV and $Pb+Pb$ at $160$ AGeV are also
presented.
