We have calorimetrically measured the heat of reaction of vapour deposited calcium atoms on a poly[2-methoxy,5-(2'-ethylhexyloxy)-p-phenylene-vinylene] (MEH-PPV) substrate. We find that Ca has an initial heat of reaction of 360 kJ/mol, and that the reaction heat decreases rapidly with coverage. In contrast, on poly[2-methoxy,5-(2'-ethylhexyloxy)-p-phenylene], which lacks the vinylene group, we find the calcium has an initial reaction heat of only 200 kJ/mol, and that the reaction heat initially rises with coverage. We conclude that the vinylene group is the dominant reaction site for calcium on MEH-PPV. (C) 2003 Published by Elsevier B.V.