Conventional phosphonate-type scale inhibitors (SIs) are commonly applied for barite-scale prevention in oil fields. The barite forms when the injection water (IW), which is usually sulfate rich, is injected into a barium-containing formation water (FW). The inhibition efficiency (IE) of barite-scale inhibitors is affected by the barium sulfate saturation ratio (SR) of the brine mix and, additionally, by the presence of divalent cations Ca 2þ and Mg 2þ . What is less well known is that the precise balance between these factors (SR and Ca 2þ /Mg 2þ ratio) can vary significantly for different phosphonate species. This paper presents novel IE experimental results for phosphonate-scale inhibitors DETPMP and HMTPMP (penta-phosphonates), OMTHP (hexa-phosphonate), and HMDP (tetra-phosphonate). Minimum-inhibitor-concentration (MIC) levels for each SI are established by testing a wide range of brine IW/FW mixing ratios, which changes (i) barite SR and precipitated mass, (ii) molar ratio of Ca 2þ /Mg 2þ , and (iii) the ionic strength of the brine mix.The phosphonate SIs were categorized into two types on the basis of their MIC-vs.-percent-North Sea-seawater (NSSW) behavior. Type 1 (e.g. DETPMP and OMTHP) are affected principally by SR and are rather less sensitive to Ca 2þ /Mg 2þ ratio although they do show some sensitivity to the latter factor. Type 2 (e.g., HMTPMP and HMDP) are severely affected by brine Ca 2þ /Mg 2þ ratio as well as SR. To demonstrate these effects conclusively, a series of IE experiments is presented with varying [Ca 2þ ] and [Mg 2þ ] (which normally occurs in the field as the IW/FW ratio changes over time) and then similar experiments are repeated at a fixed Ca 2þ /Mg 2þ molar ratio. The MIC level measured for both types of phosphonate SI always correlates very well with the barite SR at fixed Ca 2þ / Mg 2þ molar ratio (fixed case). In addition, we find the MICs of both types of SI are much lower in the fixed-case experiments (higher Ca 2þ /Mg 2þ molar ratio), compared with the base case, because of the beneficial effect of higher [Ca 2þ ]. The effects observed are important for field application of phosphonate SIs because they show how the various species are sensitive to the changing scaling problems as the %NSSW increases, in terms of SR and Ca 2þ /Mg 2þ molar ratio. These results also give some important insights into the mechanism by which different phosphonates actually work in inhibiting barite scale.