The gas-phase proton affinity of acetamidine has been determined using FT-ICR spectrometry to be 230.1 kcal/mol (962.7 kJ/mol). High-level ab initio calculations were performed to investigate whether acetamidine is planar in its equilibrium conformation. The effect of solvation on both its structure and its basicity was analyzed using the self-consistent reaction field theory. The two stable isomers 1a and 1b are almost degenerate but separated by a significantly high isomerization barrier, Much higher is the tautomerization barrier between acetamidine 1a and 1,1-diaminoethylene (2). The amino rotational barrier is of the same order of magnitude as those of typically Y-conjugated systems such as guanidine. Most unexpected, this barrier is twice as large for protonated acetamide as for protonated guanidine. This is consistent with the fact that the intrinsic basicity of acetamidine is only 3.0 kcal/mol lower than that of guanidine. In solution, the situation is similar, and acetamidine is slightly less basic than guanidine. Similarly, 1,1-diaminoethylene is predicted to be a very strong carbon base in the gas phase with a proton affinity 6 kcal/mol greater than that of guanidine.