A new compound with implications in scale and corrosion control has been isolated and its solubility measured under varying conditions of temperature and ionic strength from 25 to 75 degrees C and from 1 to 3 M ionic strength. Ferrous phosphonate was formed using the phosphonate nitrilotris(methylene phosphonic acid) (NTMP) and was found to have the stoichiometry Fe2.5HNTMP. Using a complexation and speciation model, the stability constants for the complexation of iron(II) with phosphonate were calculated, and the solubility product constant was derived for each temperature and ionic strength; at 25 degrees C and 1 M ionic strength, K-sp 10(-31.2). The temperature (T, K) and ionic strength (I, M) dependence of the negative logarithm of the ferrous phosphonate solubility product constant (pK(sp)) was determined to be : pK(sp) = 39.54 - 6.14I(1/2) + 2.8I - 1315/T. In simulated calculations using actual field data to compare iron and calcium phosphonates independently, ferrous salts were predicted to form in all instances and matured calcium salts were predicted to form occasionally. Furthermore, a relationship was established whereby the concentration of free iron in a calcium-iron-phosphonate system can be predicted for a given system.