The ionisation of pentamethylphenol in concentrated methanolic sodium methoxide solutions parallels an H_– acidity function which deviates less from ideality than the corresponding function deduced from the ionisation of t-butyl phenols. The results are consistent with the possibility that steric inhibition of solvation of the phenol anions by bulky ortho substituents is reflected not only in the acid ionisation constants of the phenols but also in the variation of acidity functions with sodium methoxide concentration.