The relative stabilities of the two anhydrous polymorphs and the monohydrate of piroxicam in ethanol/water mixtures and the crystallization behavior of the solid forms in the presence of 11 additives were studied. At 37°C, formII is thermodynamically more stable than formI, while the relative stability of formII and the monohydrate depends on the water concentration in the solution. The total Hansen solubility parameter δ_T and the hydrogen bonding solubility parameter δ_H of the substances can partly explain the effects of the additives on the crystallization of the solid forms. The additives that selectively promoted the crystallization of formI are the ones with δ_T and δ_H parameters similar to those of piroxicam.