The ground-state hyperfine splitting of trapped Cd-113(+) ions has bee
n measured to be 15 199 862 858(2) Hz. The transition was detected by
a laser microwave double-resonance method in the presence of He buffer
gas. The result was corrected for the second-order magnetic-field dep
endence. This paper also discusses the possibility of using Cd+ ions a
s a microwave-frequency standard.