H/D kinetic isotope effect has been employed to study the mechanism of the thermally assisted photocatalytic hydrogen production over noble metal-free Ti@TiO$_2$ core–shell nanoparticles. We have found that the observed large H/D isotope separation factor (αH = 11.3 ± 1.7–6.2 ± 0.9) is due to the electron hole-mediated cleavage of OH bond. It was concluded that strong H/D isotopic selectivity is associated with significant photothermal effect.