Electrocaloric (EC) effects are typically studied near phase transitions in ceramic and polymer materials. Here, we investigate EC effects in an inorganic salt, namely ammonium sulfate (NH 4 ) 2 SO 4 , with an order–disorder transition whose onset occurs at 223 K on cooling. For a single crystal thinned to 50 μm, we use a Maxwell relation to find a large isothermal entropy change of 30 J K −1 kg −1 in response to a field change of 400 kV cm −1 . The Clausius–Clapeyron equation implies a corresponding adiabatic temperature change of 4.5 K. This article is part of the themed issue ‘Taking the temperature of phase transitions in cool materials’.