The thermal stability and acidity of 12-tungstophosphoric acid (HPW) and its salts, both fresh and postreaction, have been studied by in situ and ex situ infrared and thermal analytical methods. The relationship between acidity and the lifetime and deactivation behavior in the high-pressure oligomerization of propene has also been investigated as has been the location of the active sites in heteropoly anions. These studies have confirmed a correlation between structural stability and the salt cation, with the potassium salt of HPW being the most thermally stable. A correlation has also been established between salt Type (A or B) and the nature of coke deposits formed during reaction. Type A salts produced only Type I(aliphatic) coke during butene isomerisation, butane cracking and short time-on-stream isothermal propene oligomerisation reactions but both Type I and Type II (aromatic) coke in long time-on-stream and/or nonisothermal oligomerisation operation. Type B (high surface area) salts always formed both Type I and Type II coke. In all cases the formation of Type II coke was responsible for catalyst deactivation. This coke blocking of acidic sites Cas confirmed by NH3 adsorption/desorption studies of postreaction heteropoly anions. On the basis of shifts in IR bands, it is proposed that the coke deposits responsible for deactivation are formed in the so-called cup M-O-M edge sites of the Keggin Unit. A mechanism is proposed to describe the thermal decomposition of HPW.