Experimental measurements are presented of the thermal conductivity (λ) and the density (ρ) of several clay pastes used in pelotherapy (peloids). Up to 22 peloids were tested. They were prepared by adding various amounts of water to seven mineralogically different powder clays or clay minerals. λ of each clay paste was measured at six different temperatures ranging between 293 K and 318 K, while ρ was measured at ambient temperature. A clear correlation was established between the density and the thermal conductivity of the peloids.