Ammonium sulfate exhibits a first-order phase transition upon cooling through ~ 224 K at room pressure, from a paraelectric to a ferroelectric crysta. There are outstanding questions concerning the temperature evolution of the strain through the transition, particularly at non-ambient pressures. 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 4.2 < T < 290 K, and 0 < P < 5.5 kbar. These data will allow us to establish the structural changes which occur through the phase transition, and distinguish if the process occurs in one or more steps. The data will yield a well determined isothermal equation of state at room temperature, confirm the slope of the phase boundary in P,T space, and provide precise thermal expansion coefficients at a series of pressures.