The guanidine 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) reacted with the heterocumulenes CO2, CS2, iPr2NCNiPr2, ptolylNCO and ptolylNCS to give three types of products: TBD-CO2 (1) and TBD-ptolylNCS (5), resulting from addition of the cumulene to TBD, TBD(-H)({iPrNH}-C=N{iPr}) (3) and TBD(-H)(ptolylNHCO) (4), resulting from addition of the N-H bond of TBD to the C=N bond of the cumulene, and dithiocarbamate salt [TBDH][TBD(-H)CS2] (2) after transfer of a proton to a second molecule of TBD. Competition reactions indicated that the order of decreasing stability of the compounds is: 4 > 5 > 1 > 2 > 3. DFT calculations showed that the relative stabilities of the products are governed by both the Lewis acidity of the heterocumulene and the Brønsted basicity of the TBD(-H)-cumulene anion, while their distinct structures are determined by the latter, with the most basic site governing the geometry of the final product.