[en] We have measured the ionic conductivities of pressed pellets of the layered compounds MUO₂PO₄ . nH₂O, and correlated the results with TGA data. The conductivities (in ohm^-1m^-1), at temperatures increasing with decreasing water content over the range 20 to 200⁰C, were approximately as follows: Li⁺-4H₂O, 10^-4; Li⁺-, Na⁺-, K⁺-, and NH₄⁺-3H₂O, 10^-4, 10^-2, 10^-4, and 10^-4; H⁺-, Li⁺-, and Na⁺-1.5H₂O, 10^-2, 10^-4, and 10^-4; Na⁺-1H₂O, 10^-5; H⁺-, K⁺-, and NH₄⁺-0.5H₂O, all 10^-5; and H⁺-, Li⁺-, Na⁺-, K⁺-, NH₄⁺-, and 1/2Ca²⁺-OH₂O, 10^-5, 10^-5, 10^-4, 10^-5, 10^-5, and 10^-6. A ring mechanism is proposed to account for the high conductivity found in NaUO₂PO₄.3.1H₂O. The accurate TGA data showed that most of the hydrates had water vacancies of the Schottky type, and should be represented as MUO₂PO₄(A-x)H₂O, where x can be between 0 and 0.3