Values for the ionization constants and the heats of ionization of phytic acid (inositol hexaphosphate) have been determined by simultaneous potentiometric and thermal titration in a titration microcalorimeter. The potentiometric results are in substantial agreement with those found by discontinuous titration; the heat liberated from pH 10.4 to 6.8 is essentially that found using a batch microcalorimeter. Ionization of phytic acid may be generally characterized as having five groups similar to pK of phosphoric acid, two groups similar to pK2 of phosphoric acid and 4 groups similar to pK4 of pyrophosphoric acid. A single group has pK ∼ 5 with ΔHi∼ -1.1 kcal/mol.