The formation and growth of sigma (σ) phase in a 2205 duplex stainless steel is monitored during an 850°C isothermal heat treatment using an in situ synchrotron x-ray diffraction technique. At this temperature, σ phase is first observed within approximately 40 seconds of the start of the isothermal heat treatment and grows rapidly over the course of the 3600 second heat treatment to a volume fraction of approximately 13%. A simultaneous increase in the austenite (γ) volume fraction and a decrease in the ferrite (δ) volume fraction are observed. The σ phase formed at this temperature is rapidly dissolved within approximately 200 seconds when the temperature is increased to 1000°C. Accompanying this rapid dissolution of the σ phase, the δ and γ volume fractions both approach the balanced (50/50) level observed in the as-received material.