This work studies the solid‐state modification of potato starch using sodium trimetaphosphate (STMP) at elevated temperature (120°C). Kinetics for the reaction was investigated using an in situ technique based on attenuated total reflectance infrared spectroscopy for monitoring the extent of crosslinking. For tested STMP concentrations between 1.6 and 20% (w/w, dry starch basis), the degree of substitution varied between 0.015 and 0.106 mol P/mol anhydroglucose unit, respectively, within a reaction time of less than 10 min. The reaction data was found to follow a first‐order rate dependency and by examination using a phase‐boundary kinetic model was considered to be exterior diffusion limited. The extent of conversion always converged to an asymptotic value less than 100%. The inclusion of 1–2% hydrocolloidal xanthan gum were found to have no effect on the crosslinking of starch whereas 10–30% glycerol significantly lowered the rate of reaction.