The aging behavior of two novel poly(ether ether ketone)s, 6F-PEEK and m-TPEEK, which were aged at 20degreesC below their glass transition temperatures, were studied by modulated differential scanning calorimetry (MDSC). The total, nonreversing heat flows and complex heat capacities of MDSC of the two kinds of materials were discussed. Similar total heat flows of MDSC to the ordinary DSC were achieved. They both grew in magnitude and moved to higher temperature as the aging time increased, which is associated with and characteristic of physical aging in heat flow. The complex specific heat capacity became sharper with aging time, which resulted in a higher T-mid. The relaxation time was calculated from the excess enthalpy. SAXS measurement added another proof that dense amorphous phase formed in the metastable glass resulted from the rearrangement of molecular segments during aging. The increasing speed of dense amorphous phase fraction X-DAP of m-TPEEK was bigger than that of 6F-PEEK, which means the molecular segments of m-TPEEK are quicker to form dense amorphous phase than that of 6F-PEEK. This conclusion is consistent with their structures. The molecular chains of 6F-PEEK with bigger pendant group are more difficult to rearrange than those of m-TPEEK. (C) 2005 Wiley Periodicals, Inc.