Ammonium sulfate exhibits a first-order phase transition upon cooling through ~ 224 K at room pressure, from a paraelectric to a ferroelectric crystal. There have been no studies of the structure, the phase transition, or the thermoelastic properties above 0.6GPa. Given the possible relevance of ammonium sulfate to the interior of Saturn's moon Titan, we propose to make powder diffraction measurements over the range 120 < T < 290 K, and 0 < P < 5 GPa using the Paris-Edingburgh cell. These data will allow us to measure the isothermal equation of state to 5 GPa, identify any polymorphic phase transtions, and investigate the P,T dependence of the para- to ferro-electric transition to much higher pressures than previously investigated.