The chemical rate constants for the addition of the muonium atom (Mu) across the vinyl double bonds of acrylamide, acrylic acid, acrylonitrile, methylmethacrylate, and styrene were determined in aqueous solution; they are, respectively, kM = 1.9 × 1010M−1s−1, 1.6 × 1010M−1s−1, 1.1 × 1010M−1s−1, 9.5 × 109 M−1s−1, and 1.1 × l09 M−1s−1. Since muonium can be considered a very light isotope of hydrogen, the kinetic isotope effects, kM/kH, for acrylamide (1.1) and acrylonitrile (2.8) were calculated. The muonium rate constants of these monomers are also compared to those of hydroxyl and methyl radical additions where available. The muonium substituted free radical formed by reaction with styrene is represented by two peaks in the Fourier Transform of the μSR spectrum at 500, 1500, and 2500 G with a hyperfine coupling constant of 213.5 MHz. This spectrum shows that Mu addition to styrene occurs at the vinyl bond only and not at the benzene ring.