Thermal lag has long been recognized as a problem in differential scanning calorimetry (DSC) and a correction procedure that assumes constant thermal resistance is often adopted for application as needed. Using various special sandwiched samples composed of high purity indium and polyethylene films, we have found that thermal resistance varies significantly with location within a polymer sample. Indium foil located near the sample surface melts earlier and most of it melts quickly, compared to indium located in the middle of the sample. Experimental data were analyzed using Gray’s theory and excellent agreement was achieved when allowing for heat capacity differences for different locations within the sample. This work provides a more comprehensive understanding of the melting process, and is essential to decoupling instrumental and sample effects.