Background: Differential cross sections for the (p, n) and (n, p) reactions on 90 Zr over the interval of 0 − 50 MeV excitation energy were used to determine the corresponding GT strengths, and the resulting quenching factor ≈ 0.9 with respect to the Ikeda sum rule. In this procedure the contribution of the isovector spin monopole (IVSM) strength was subtracted from the total strength without taking into account the interference between the GT and the IVSM modes.Purpose: To determine the quantitative effect of the IVSM excitation mode on the L = 0 strength in charge-exchange reactions on several closed-shell nuclei and the Sn isotopic chain.Method: The fully consistent relativistic Hartree-Bogoliubov (RHB) model + proton-neutron quasiparticle random phase approximation (pn-RQRPA) is employed in the calculation of transition strength in the β − and β + channels.Results: The inclusion of the higher-order terms, that include the effect of finite momentum transfer, in the transition operator shifts a portion of the strength to the high-energy region above the GT resonance. The total strength is slightly enhanced in nuclei with small neutron-to-proton ratio but remains unchanged with increasing neutron excess. Conclusions:Terms that include momentum transfer in the transition operator act mostly to shift the strength to high excitation energies, but hardly affect the total strength. Based on the strength obtained using the full L = 0 transition operator in the pn-RQRPA calculation, we have estimated the impact of the IVSM on the strength measured in the charge-exchagne reactions on 90 Zr and found that the data are consistent with the Ikeda sum rule.