AbstractIn the last few years, the superconducting transition temperature, T_c, of hydrogen-rich compounds has increased dramatically, and is now approaching room temperature. However, the pressures at which these materials are stable exceed one million atmospheres and limit the number of available experimental studies. Superconductivity in hydrides has been primarily explored by electrical transport measurements, whereas magnetic properties, one of the most important characteristic of a superconductor, have not been satisfactory defined. Here, we develop SQUID magnetometry under extreme high-pressure conditions and report characteristic superconducting parameters for Im-3m-H₃S and Fm-3m-LaH₁₀—the representative members of two families of high-temperature superconducting hydrides. We determine a lower critical field H_c1 of ∼0.82 T and ∼0.55 T, and a London penetration depth λ_L of ∼20 nm and ∼30 nm in H₃S and LaH₁₀, respectively. The small values of λ_L indicate a high superfluid density in both hydrides. These compounds have the values of the Ginzburg-Landau parameter κ ∼12–20 and belong to the group of “moderate” type II superconductors, rather than being hard superconductors as would be intuitively expected from their high T_cs.