Dielectric relaxation time of ice V at 0.6 GPa: H <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si37.svg" display="inline" id="d1e248"> <mml:msub> <mml:mrow/> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:math> O/D <m

Dielectric-loss spectra of ice V were measured in situ at 0.6 GPa for H 2 O and D 2 O. A well-resolved relaxation peak in the imaginary permittivity ɛ ′ ′ shifts to lower frequencies upon cooling, enabling direct determination of the peak frequency f p . Characteristic relaxation times were obtained. The present dataset covers 243–153 K for H 2 O and 243–163 K for D 2 O in the frequency window 10 mHz–1 MHz. The relaxation time of ice V with H 2 O reproduces the Johari and Whalley high-pressure reference at high temperatures but deviates systematically at lower temperatures, while that of D 2 O ice V is shifted to longer relaxation times by ∼ 0.2 decades at the high-temperature end and by ∼ 0.5 decades toward the low-temperature end. The modest in situ isotope effect contrasts with the stronger response reported for ambient-pressure recovered samples. • Dielectric-loss spectra of ice V were measured at 0.6 GPa for both H 2 O and D 2 O. • Systematic low-T deviation from Johari–Whalley reference identified. • Modest isotope effect (∼0.2–0.5 decades) unlike 1-bar recovered samples.