Duplex stainless steels are desirable for use in power generation systems due to their attractive combination of strength, corrosion resistance, and cost. However, thermal embrittlement at intermediate temperatures (~475°C) via α-α' phase separation limits upper service temperatures for many applications. The development of low Cr and Ni equivalent lean grade alloys potentially increases the upper service temperature of these alloys by delaying the onset of α-α' phase separation. The present work assesses the thermal stability of a relatively new lean grade of duplex stainless steel, alloy 2003. In this paper, alloy 2003 has been compared to the most widely used duplex alloy, 2205, through a series of isothermal agings between 260°C and 538°C for times between 1 and 10,000 hours. The thermal stability of these alloys was primarily characterized by changes in microhardness. The microhardness data were fit to a JMA-type equation to quantify embrittlement rates and predict microstructural changes out to 50 years. Additionally, as-received specimens were characterized with the scanning electron microprobe to quantify the chemistry within the ferrite grains relative to the bulk material. Alloy 2003 was shown to be much more resistant to thermal embrittlement than alloy 2205. For 50 years of service at 288°C, it is predicted that alloy 2003 components will have a change in microhardness of about 25 HK where alloy 2205 components would increase approximately 175 HK, which indicates significant embrittlement. These findings show that lean grade alloys will have a greater service temperature range than standard grades. However, additional data, characterization, and modeling are needed to better predict embrittlement kinetics over component lifetimes.