  Thermochemical equilibrium and kinetic calculations are used to model sulfur
and phosphorus chemistry in giant planets, brown dwarfs, and extrasolar giant
planets (EGPs). The chemical behavior of individual S- and P-bearing gases and
condensates is determined as a function of pressure, temperature, and
metallicity. The results are independent of particular model atmospheres and,
in principle, the equilibrium composition along the pressure-temperature
profile of any object can be determined. Hydrogen sulfide (H2S) is the dominant
S-bearing gas throughout substellar atmospheres and approximately represents
the atmospheric sulfur inventory. Silicon sulfide (SiS) is a potential tracer
of weather in substellar atmospheres. Disequilibrium abundances of phosphine
(PH3) approximately representative of the total atmospheric phosphorus
inventory are expected to be mixed upward into the observable atmospheres of
giant planets and T dwarfs. In hotter objects, several P-bearing gases (e.g.,
P2, PH3, PH2, PH, HCP) become increasingly important at high temperatures.
