The effect of hydrate inhibitors on oilfield scale formation has been studied. A self-consistent activity model is presented to model the effect of methanol on carbonate equilibrium, calcite, barite, gypsum, celestite, and halite solubility in a gas/methanol/water/salt solution. The model uses Pitzer theory to model the effect of salt and Born equation to model the effect of methanol. Excellent agreements between the model prediction versus both experimental and literature results are observed. The model has been incroporated into ScaleSoftPitzer®, a scale prediction software specifically written for the oil and gas industries. From ScaleSoftPitzer® predictions, scaling problems can be worsened with as little as 5% (by volume) of methanol in a typical oil and gas production system. Ethylene glycol has a much lesser impact on oilfield scale formation than methanol. The efficiency of common phosphonate inhibitors in the methanol and ethylene glycol containing brines will also be discussed.