Information on how cosolvents affect sorption of ionizable chemicals by soils with heterogeneous variable-charge mineral surface domains is sparse. In this study, the effect of soil–solution pH in methanol/water solutions on sorption of pentachlorophenol (PCP) by variable-charge soils with a range of hydrophilic (f Hphilic ) and hydrophobic (f Hphobic ) sorption domains was characterized. PCP sorption by 10 variable-charge soils was measured as a function of apparent pH (pH app ) and methanol volume fraction (f c ⩽0.6). Sorption data of both neutral (pH app <3) and anionic PCP (pH app >8) decreased log-linearly with increasing f c , but the slope of the relationship was less for anionic PCP. The empirical solvent–sorbent interaction term for anionic PCP (α i ) was inversely correlated with f Hphilic (r 2 =0.82), which is consistent with methanol-induced increases in anion exchange. For neutral PCP, the empirical term (α n ) was positively correlated with f Hphobic (r 2 =0.84), supporting methanol-induced increases in solution and sorbent hydrophobicity. Sorption of PCP by two soils with varying f Hphilic in the pH app range from 3 to 8 at f c ⩽0.5 was well predicted using a cosolvency model. The model included only pHapp-pKa′ dependent speciation and the different intrinsic hydrophobic partitioning of neutral and anionic PCP without explicitly incorporating the contribution of f Hphilic . Therefore, although the degree to which methanol affects sorptivity of the two PCP species differs with sorption domain types, incorporating both cosolvent-enhanced solubility and cosolvent-induced speciation is sufficient to adequately predict the overall sorption on variable-charge soils.