Lanthanide(II) reagents (SmI2, SmBr2, YbI2, EuI2) have been prepared rapidly in high yields using microwave‐assisted heating. Samarium diiodide is obtained as a saturated solution in THF (0.13 M) within 5 min and ytterbium diiodide (0.065 M) after 45 min. A simple method for quantitative determination of LnX2 in THF is also described by utilizing the instantaneous reaction with ketones in the presence of an amine and water. This method establishes the concentration of the active single‐electron‐donor species, i.e. Ln2+. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)