Abstract Large volumes of hydrate inhibitors (e.g., methanol, ethanol, monoethylene glycol, and triethylene glycol as cosolvent) are added to control hydrate formation. Such practice has an adverse effect on scale formation since the mineral salts are generally less soluble in the cosolvent. Due to production from reservoirs, oilfield brines are often close to saturation as they enter a well, even a small amount of added methanol, ethanol, etc., is often sufficient to induce various minerals to precipitate, particularly the sparingly soluble minerals, e.g., barite. For example, barite solubility is reduced by as much as 20 folds, with 50% (w/w) methanol. In this paper, barite nucleation rate were studied over a wide range of concentrations, e.g., Ba (0.5–1.8 mm) SO4 (0.5–1.8 mm) methanol (0–40%), monoethylene glycol (0–40%) or triethylene glycol (40%). Barite nucleation rate is significantly accelerated in as little as 5% (wt/wt) methanol. The barite nucleation rate can be modeled with a modified classical nucleation theory. The inhibition of barite by two phosphonate inhibitors and a polymer inhibitor in the co-solvent/brine solution is more complex. At lower co-solvent concentrations (<30% w/w), the nucleation inhibition can be predicted with a previously derived semi-empirical model that mathematically separates the effect of added inhibitors from that of the uninhibited mineral phase. At high methanol (>30%) concentration, barite nucleation may be difficult to inhibit by scale inhibitors, due to high supersaturation and the tendency of phosphonate to be precipitated as metal salt.