We report the results from an investigation into the structural evolution of a potential new thermoelectric material, Cu3SbSe3, as a function of temperature from 25 to 390°C. From high-temperature X-ray diffraction data, the refined lattice parameters were seen to change non-linearly, but continuously, with temperature, with an increased rate of thermal expansion in the a and b lattice parameters from around 125°C until around 175°C and negative thermal expansion in the c axis from around 100°C until around 175°C. Crystallographic charge flipping analysis indicated an increase in the disorder of the copper cations with temperature. This reversible order/disorder phase transition in Cu3SbSe3 affects the transport properties, as evidenced by thermal diffusivity measurements, which change from negative to positive slope at the transition temperature. This structural change in Cu3SbSe3 has implications for its potential use in thermoelectric generators.