Transmission of substituent effects through 5-membered heteroaromatic rings is investigated theoretically at the RHF/6-31+G and B3LYP/6-31+G levels using the equilibria for the $OH^- addition to five-membered heteroaromatic aldehydes (5MHA). The transmission efficiency (S) in 5MHA(A) increases in the order NH>O>S>PH but the order exactly reverses in 5MHA($T^-$). This is originated by the delocalizability of the ${pi}$ lonepair on Y, $$n_pi$(Y)$</TEX>. A better correlation is obtained with ${sigma}_p^-$ in the Hammett plots with positive slope, $p_z$ > 0, indicating that the substituent (Z) effects are not transmitted by a direct conjugation. The magnitude of $p_z$ for Y=NH is the largest among the heteroaromatic systems, which is consistent with the largest transmission efficiency change $({Delta}S)$. The equilibria for the addition processes are favorable in the gas phase $({Delta}G^o<0)$, which reverses to unfavorable in aqueous solution $({Delta}G^o>0)$ due to the relatively large salvation energy of $OH^-$ in the initial state in aqueous solution. The orders of ${Delta}G^o$ and $p_z$ in the gas phase are almost maintained in solution.