The solid acidity of 12-tungstophosphoric acid (H3PW) has been measured using slurry calorimetry in cyclohexane with pyridine as the probe. Analysis by the Cal-ad method, pyridine adsorption slurry calorimetry coupled with a pyridine adsorption isotherm, shows the protons of solid H3PW have different strengths. The two different types of acidic protons (sites) in H3PW have populations, n, and enthalpies of interaction, Delta H, with pyridine in a cyclohexane slurry of n(1) = 0.08 mmol g(-1), Delta H-1 = -32.7 kcal mol(-1), and n(2) = 0.16 mmol g(-1), Delta H-2 = -19.6 kcal mol(-1). The total number of protons titrated (0.24 mmol g(-1)) confirms that not all of the available protons of H3PW (1.04 mmol g(-1)) react with pyridine. These results suggest that the total number of protons titrated could be assigned to the number of protons in every even plane of the lattice (E00). The different protons were assigned to the face (n(1)) and edge (n(2)) positions in the unit cell of H3PW. The greater number of protons titrated with pyridine (0.24 mmol g(-1)) than the number of surface protons (0.008 mmol g(-1)) for H3PW illustrates penetration of pyridine into the lattice. X-ray diffraction results demonstrate expansion of the H3PW lattice in which sites 1 and 2 are titrated with pyridine. The observation of single-peak resonances in P-31 magic-angle spinning NMR of H3PW-pyridine adducts demonstrates that proton mobility distributes the anionic charge throughout the sample. The acid sites of H3PW are confirmed as Bronsted type by Fourier transform infrared spectroscopy. The enthalpies of the most acidic proton measured for solid H3PW (-32.7 kcal mol(-1)) and for H3PW dissolved in acetonitrile (-30.4 kcal mol(-1)) are in good agreement.