The parity-violating decay of muons in a storage ring is observed. The difference frequency $\omega _{\mathit a}$ between the muon spin precision and the orbital angular frequency (${{\mathit e}}$ /${\mathit m}_{{{\mathit \mu}}}\mathit c)\langle{}\mathit B\rangle{}$ is measured, as is the free proton NMR frequency $\omega _{{{\mathit p}} }$, thus determining the ratio $\mathit R=\omega _{\mathit a}/\omega _{{{\mathit p}} }$. Given the magnetic moment ratio $\lambda =\mu _{{{\mathit \mu}} }/\mu _{{{\mathit p}} }$ (from hyperfine structure in muonium), ($\mathit g−$2)/2 = $\mathit R/(\lambda −\mathit R$).