Abstract.We provide an accurate evaluation of the two-photon exchange correction to the hyperfine splitting of S energy levels in muonic hydrogen exploiting the corresponding measurements in electronic hydrogen. The proton structure uncertainty in the calculation of α 5 contribution is sizably reduced.The theoretical knowledge of the two-photon exchange (TPE) correction to the hyperfine splitting (HFS) of the S energy levels in muonic hydrogen exceeds by two orders of magnitude the expected ppm level of the experimental accuracy in the forthcoming measurements of 1S HFS by CREMA [1] and FAMU [2,3] Collaborations as well as at J-PARC [4]. In the ordinary hydrogen, the uncertainty of TPE is even six orders of magnitude above the experimental precision [5,6], the measurements were performed in the 1970s [7][8][9][10][11][12][13][14][15] and discussed in refs. [16,17].The graph with two exchanged photons, see fig. 1 for the notation of particles momenta, also contributes the largest theoretical uncertainty in the proton size extractions from the Lamb shift in muonic hydrogen (μH) [18,19]. It was a subject of extensive theoretical studies [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] since the formulation of the proton radius puzzle in 2010, when the accurate extraction of the proton charge radius (R E ) from the muonic hydrogen Lamb shift by the CREMA Collaboration at PSI [18,19] gave a significantly smaller result than the electron data based extractions [35][36][37], see refs. [19,38] for recent reviews.Besides the Lamb shift, the CREMA Collaboration has extracted the HFS of the 2S energy level in μH [39], where the leading theoretical uncertainty is also coming from TPE. The corresponding correction to HFS of S energy levels is expressed in terms of the proton elastic form factors and spin structure functions [5,[40][41][42][43][44][45][46][47][48][49][50][51][52]. The first full dispersive calculation of this contribution was performed in refs. [5,49], where it was evaluated with 213 ppm uncertainty. The subsequent studies expressing the region with small photons virtualities in terms of proton radii and moments of the spin structure functions led to the uncertainty 105 ppm [53]. The model-independent evaluation within the frameworks of Non-Relativistic Quantum a e-mail: tomalak@uni-mainz.de Electrodynamics and Chiral Perturbation Theory exploiting the electronic hydrogen (eH) HFS measurement was recently performed in ref.[51], for results in Chiral Effective Field Theory see ref. [50].In this work, we aim to reduce the proton structure uncertainty in the dispersive evaluation of the TPE correction to HFS of the S energy levels in μH exploiting precise measurements of the HFS in eH [16].The TPE contribution to the nS-level HFS δE HFS nS is expressed in terms of the relative correction Δ HFS and the leading-order HFS E HFS,0 nSwhere M and m are the proton and the lepton masses in the energy units, m r (m) = Mm/(M + m) is the reduced mass, μ P is the proton magnetic moment, α is the electromagnetic ...