The halide ion mobility in mixed halide perovskite exhibits two opposite trends in response to photo and thermal activation. While halides prefer to remain as Br-rich and I-rich domains under steady state light irradiation of MAPbBr1.5I1.5 films, they prefer to remain in their stable mixed composition when kept in the dark. The activation energies as determined from the temperature-dependent rate constants are Ea,forward=28.9 kJ mol-1 for photoinduced segregation and Ea,reverse=53.5 kJ mol-1 for remixing of halides in dark respectively. The energy input from photoexcitation assists to overcome the dark (thermally activated) mixing to induce Br-rich and I-rich domains. This segregated state is maintained as long as the mixed halide film is irradiated continuously with visible light. The excitation intensity threshold above which segregation occurs follows a linear temperature dependence, such that phase separation occurs above Iexc=30 μW/cm2 white light at 23˚C. The threshold at 90 ˚C becomes higher with minimum intensity requirement of 100 μW/cm2 to induce segregation. The thermodynamic rationale behind this unusual halide mobility under photo and thermal excitation discussed here can aid in understanding the stability issues of perovskite solar cells.