Amine-silica adsorbents are considered alternatives to aqueous solutions of amines, which have been traditionally used to capture carbon dioxide (CO2) from flue gas. Among amine-silica adsorbents, polyethyleneimine (PEI)-silica is particularly effective at capturing CO2 from flue gas due to its high thermal stability. In this study, we investigated the influence of PEI type (i.e. branched vs. linear) and molecular weight on the CO2 capture performance of PEI-silica adsorbents. PEI molecular weight influenced the thermal stability of PEI-silica adsorbents; however, when the molecular weight was >= 1200 Da the increase in stability was negligible in the temperature range of 25-160 degrees C. Branched PEIs (BPEIs) achieved higher CO2 saturated sorption capacities compared to linear PEIs (LPEIs); however, LPEIs were more stable than BPEIs during CO2 sorption-desorption cycling. PEI molecular weight also influenced the CO2 saturated sorption capacity; CO2 saturated sorption capacity decreased as PEI molecular weight increased, and among the adsorbents tested in this study BPEI/800-silica had the highest CO2 saturated sorption capacity (202 mg CO2/g adsorbent). Both PEI type and molecular weight exhibited influence on the sorption or desorption heat of PEI-silica adsorbents. The CO2 regeneration heat was much lower than that of MEA solution for all PEI-silica adsorbents tested in this study. (C) 2014 Elsevier Ltd. All rights reserved.