The phase separation behavior of polymer blend thin films of 100-150 nm was studied using differential AC Chip calorimetry. By taking advantage of the low sensor and sample mass inherent to chip calorimetry, a new methodology based on temperature jumps was developed. This methodology allowed the construction of the state diagram of polymer blend thin films as evidenced for two model systems (PVME/PS and PVME/Phenoxy) displaying a lower critical solution temperature behavior. The state diagram in thin films was compared to the one obtained in bulk using Modulated Temperature DSC. In comparison with bulk, a lower phase separation temperature and a broadening of the homogeneous glass transition temperatures is observed for both model systems. This might be an indication of a surface induced 'destabilization' by composition gradients which are not present in bulk. (C) 2011 Elsevier Ltd. All rights reserved.