Purpose – This paper sets out to observe the changes occurring during thermal processing of Na2CO3 and FePO4 · 2H2O in control trials, then analyses the influence on availability of phosphorus compounds in comparison with pure FePO4 · 2H2O.Design/methodology/approach – Control trials of Na2CO3/FePO4 · 2H2O with molar ratios in the range 0.5‐5.0 were heated to a temperature of 850°C, then calcined within 3 hours without mixing. Cooled products were submitted for chemical analysis regarding the content of phosphorus soluble in cold water, 0.4 percent HCl, 2 percent citric acid and aqua regia as well as crystallographic identification.Findings – The addition of Na2CO3 significantly increases the availability of phosphorus compounds from FePO4 · 2H2O. The total amount of phosphorus in control trials after the calcining process is soluble in cold water, citric acid and HCl. The molar ratios used affect the type of crystalline phase formation only in the case of a molar ratio near to 1.0. Near‐perfect phosphoru...