The radioactive mercury isotopes produced by the bombardment of mercury with neutrons and by the bombardment of gold with deuterons have been studies. Two activities with half-lives of 23 hours and 64 hours, respectively, both decaying by $K$-electron capture, have been assigned to ${\mathrm{Hg}}^{197}$, on the basis of critical absorption measurements made on the x-rays emitted. The same method was used to show that the 43-minute activity produced by fast neutrons in mercury belongs to an excited state of a mercury isotope and decays by an isomeric transition to the ground state. A 51.5-day beta-emitter was studied; it is probably to be assigned to ${\mathrm{Hg}}^{203}$.