The apparent heat capacity of polyethylene fibers in the melting region was measured by quasi-isothermal, temperature-modulated differential scanning calorimetry (TMDSC) and compared with results from standard differential scanning calorimetry (DSC) and the solid and liquid thermodynamic heat capacity as references from the ATHAS Data Bank. Using a multi-frequency, complex sawtooth modulation in the quasi -isothermal mode disclosed for the first time that the uncorrected apparent heat capacity C-p = A(phi)(A(T delta)omega) of the liquid polyethylene fiber increases with increasing frequency (A(phi) is the differential heat-flow rate and AT, is the sample temperature). The frequency-dependent heat capacity cannot be represented by the expression: C-p = A(phi)(A(T),v omega)[1 + (tau v omega)(2]0.5) because of a negative tau(2). The results were later confirmed by independent measurements on single sinusoidal quasi-isothermal TMDSC on the same material. The error is caused by shrinking of the fiber, which deforms the sample pan. (c) 2005 Elsevier B.V. All rights reserved.