The present investigation applies laser Raman spectroscopy under in situ conditions to obtain insights on the effect of the temperature on the molecular structure of the bulk phosphotungstic Wells-Dawson heteropoly-acid H(6)P(2)W(18)O(62)center dot xH(2)O (HPA). The in situ temperature-programmed studies followed the evolution of phosphotungstic Wells-Dawson and Keggin heteropoly-acids along with tungsten trioxide under controlled atmosphere and temperature. The spectroscopic investigation of the Wells-Dawson HPA demonstrated that in situ Raman spectroscopy is a suitable technique to follow the effect of a gradual dehydration on the secondary structure of such a complex structure. Moreover, the absence of the signals belonging either to the Keggin or WO(3) phases provides further evidence that the phosphotungstic heteropolyanion does not decomposes towards those materials at temperatures below 600 degrees C. (C) 2011 Elsevier Ltd. All rights reserved.