A cost-effective general procedure is proposed to calculate the electron affinities of covalent linkages, which can attach an electron to create a three-electron bond. The procedure is based on a thermodynamic cycle, of which each step is calculated by a method that is specific to it. In the present work, the first step, the dissociation of a two-electron bond, is studied at the MP4 level with a correction for basis set effects, which is calculated at the MP2 level, and an empirical correlation correction that is routinely used in the Gaussian-2 scheme. Experimental data or density functional theory can be used for the second step, while the third step, the formation of a three-electron bond, is studied at the MP2 level in large basis set. ZPE corrections are included in steps 1 and 3. The method is applied to some alkyl-substituted disulfides and yields results that are in good agreement with reference values when available. Some ways to make the procedure even more economical and applicable to large molecules are discussed.