Proton resonance spectra are reported for mixtures of methyl- and ethyl-zinc iodide with the corresponding bisalkylzincs in tetrahydrofuran solution. At low temperatures, alkyl exchange is slow on a proton resonance timescale. For the Schlenk equilibria R₂Zn + Znl₂⇌ 2RZnl K is 500 at –70 °C, in agreement with previous results.