Cs 2.5H 1.5PV 1Mo 11− x W x O 40 Keggin-type polyoxometallate (POM) compounds have been synthesised and studied for selective propane oxidation in the 300–400 °C temperature range. Prior to reaction the samples were pre-treated at either 300 °C or 400 °C in order to change the concentration in Brønsted acid sites by decreasing the amount of constitutional water. Acid strength was enhanced by substituting increasing amounts of W 6+ in the Keggin anion for Mo 6+, between 0 and 6 per Keggin Unit (KU) as shown by NH 3-TPD of the H + form and IR of the lattice vibrational modes (M O and M O M with M = Mo 6+ or W 6+). As a matter of fact vibrational mode frequencies and thus the corresponding bond energies were observed to increase with W 6+ substitution. This results in more covalent M O bonds and thus to freer protons. Chemical analysis, IR and DTA/TG data allowed us to determine the extent of W 6+ substitution for Mo 6+, the amount of constitutional water and any structural change in the samples. It was observed that under catalytic conditions, (C 3/O 2/He = 2/1/2, flow rate 15 cm 3 min −1, 12 h on stream, reaction temperature in the 300–400 °C range) the catalyst structure was maintained, with only a very small part of the substituted elements (V 5+, W 6+ and Mo 6+ atoms) being extracted from the Keggin anion. Catalytic data have shown that introduction of W 6+ to replace Mo 6+ led to lower propane activation, which may be due to a decrease of lattice oxygen anion mobility, related to the stronger M O bond (see supra and as shown by the higher reaction temperature necessary for the same conversion level). Compared to pre-treatment temperature at 300 °C, pre-treatment at 400 °C was observed to result in a higher extent of constitutional water removal i.e. a loss of Brønsted acid sites by a factor of 2–4.5 when W 6+ amount varies from 0 to 6 per KU without destroying the primary Keggin structure, and to favour the formation of propene at the expense of acetic and acrylic acids and CO x . This also shows that substituting W 6+ for Mo 6+, which enhances Brønsted acid strength is detrimental to propene formation and leads to higher selectivity to acetic acid and CO x , i.e. the propane oxidation pathway via the isopropanol route and acetone rather at the expense of the usual main pathway forming acrylic acid via propylene.