Development of sustainable flame retardants for cotton-based materials has become an active research interest as many of the currently used flame retardants are harmful both to the environment and to human health. Phytic acid is a natural resource that has shown potential to be an environment-friendly flame retardant. In order to design highly efficient flame retardants based on phytic acid, detailed knowledge of the decomposition process is essential. In this study, cotton samples were prepared with pure phytic acid, as well as salts of sodium or calcium ions. All samples improved the fire performance, most notably sodium phytate, as shown by a simple burning test, as well as by thermogravimetric analysis. The formation of char prevents the degradation of cotton, leaving a residual mass. Phosphorus-31 nuclear magnetic resonance spectroscopy was used to characterize the intermediate products formed during the pyrolysis process. Both phytate salts polymerize, sodium to a higher degree than calcium.