Volatility is an important issue for aqueous ammonia used as an absorbent for post-combustion CO 2 capture. Vapor–liquid equilibrium (VLE) of NH 3 in aqueous ammonia and aqueous blends of ammonia and piperazine (PZ) were measured at 20°C, 30°C and 40°C. The experimental VLE results for NH 3 cover the mole fraction ranges 2.5–15% NH 3 (aq) in aqueous ammonia and 2.5–15% NH 3 in the 0.4M PZ blended solutions. Vapor pressures of NH 3 for 5%, 10% and 15% NH 3 (aq) were compared with available data in the literature. Activity coefficients for NH 3 in NH 3 (aq) and aqueous blends of NH 3 +PZ were calculated and compared for different temperatures to obtain the effect of PZ on the volatility of aqueous ammonia. The eNRTL model was used to model the results and a comparison between model predictions and experimental data shows good agreement.