AC chip calorimetry is used to study the phase separation behavior of 100 nm thin poly(vinyl methyl ether)/poly(styrene) (PVME/PS) blend films. Using the on-chip heaters, very short (10 MS-10 S) temperature jumps into the temperature window of phase separation are applied, simulating laser heating induced patterning. These temperature pulses produce a measurable shift in the glass transition temperature, evidencing phase separation. The effect of pulse length and height on phase separation can be studied. The thus phase separated PVME/PS thin films remix rapidly, in contrast with measurements in bulk. AC chip calorimetry seems to be a more sensitive technique than atomic force microscopy to detect the early stages of phase separation in polymer blend thin films. (C) 2010 Elsevier Ltd. All rights reserved.