Data are presented on the heats of phase changes and heat capacities for the even chain number mercury(II) carboxylates from octanoate to octadecanoate. The octanoate and dodecanoate melt directly to the liquid, while the decanoate and octadecanoate pass through a solid → solid transition before melting to the liquid. The tetradecanoate and hexadecanoate pass through a mesophase before finally melting. It is proposed that this mesophase is aG (smectic) phase. Addition of mercuric oxide to the tetradecanoate and hexadecanoate causes the appearance of an additional mesophase. The results are explained in terms of theR theory of fused micellar phases.