We have applied the temperature jump method to the study of the properties of the electrochemical double layer formed in contact with platinum stepped surfaces vicinal to the (111) orientation. High power laser light (similar to1 MW cm(-2)) has been used to increase the surface temperature (by ca. 15 K) of the electrode suddenly over very short time periods (similar to10 ns). The resulting change of the open circuit potential has been monitored and analysed in terms of the entropies of double layer formation. The observation of two potentials where the change of the open circuit potential induced by the laser illumination is zero indicates the existence of two local maxima in the entropy of the formation of the double layer. We explain this inference in terms of the existence of cooperative effects in the formation of the structure of the adsorbed water layers, which are disrupted at the potentials where the local contributions of the step and the terrace sites to the overall entropy of formation are maximal. (C) 2003 Elsevier B.V. All rights reserved.