Barium sulfate and/or calcium carbonate scale inhibitor composition composed of a water-soluble polymer having incorporated phosphate functionality, the polymer being formed from at least one ethylenically unsaturated carboxylic acid monomer, at least one ethylenically unsaturated vinyl sulfonate monomer, or a mixture thereof. The scale inhibitor composition can be used as a means of detecting inhibitor composition in downhole and topside oilfield treatments by means that are faster and more accurate than turbidometric measurement.