Photometric titration for 5-substituted 4-hydroxy-2-methylpyrimidine and its methyl derivatives was performed in n-BuOH. An equation for the titration was derived in consideration of ion pair formation in n-BuOH. [numerical formula] It was proved that Higuchi's Type II plot method was applicable only under a condition, K_BHX=K_IHX, from the analysis of the titration curves. The over-all dissociation constant of the pyrimidines (K^n-BuOH_BH=K^O_HX/K_BK_BHX=[H⁺] [B]f_H/[BH⁺] f_BH) was estimated from the slope of the titration curve. The basicities of the compounds in n-BuOH generally increased as compared with those in water (ΔpK=pK^n-BuOH_BH-pK_a=-0.1-2.3). The degrees of the increase differed largely among the compounds. Much larger increase of the basicity than the other compounds (ΔpK ; 2.1-2.3) was found for 5-substituted 1, 2-dimethyl-4(1H)-pyrimidones.