pK(a) values are estimated independently, by two entirely different methods, for the ionizations of the apical and equatorial OH groups of two representative hydroxyphosphoranes. A bond length-pK(a) correlation based on crystal structures of cyclohexanol derivatives gives values of 13.5 +/- 1.5 and 8.62 +/- 1.87, respectively, for the apical and equatorial OH groups of tetracyclohexyloxyhydroxyphosphorane, and an ab initio molecular dynamics calculation gives values of 14.2 and 9.8 for the corresponding first ionizations of pentahydroxyphosphorane.