Cationic standard heteroconjugation constants K-f(o)(BHB1) have been determined by potentiometric titration in systems formed by 4-aminopyridinium and 4-methylpyridinium cations with 4-methylpyridine, 2-methylpyridine, and pyridine in dimethyl sulfoxide. The constants, expressed as 1gK(f)(o)(BHB1), varied between 1.45 and 3.20. The most stable heterocomplex, with K-f(BHB1) = 3.20, was formed between protonated 4-methylpyridine and pyridine. A closer scrutiny of the constant values has shown that in systems with proton donors of enhanced basicity, the constants increased with declining basicity of the proton donor at a fixed proton acceptor. Before the determination of the heteroconjugation constants, acid dissociation constants were determined for the pyridinium ion and a variety of protonated pyridine derivatives in dimethyl sulfoxide. Their pK(a)(o)(BH+) values fell in the range 0.05 to 8.48 and were one to two factors of 10 lower than the corresponding pK(a)(o)(BH+) values in water. Further, it was found that cationic homoconjugation equilibria were not established in the systems involving pyridine derivatives in dimethyl sulfoxide.