Cyanoacetaldehyde (N CCH2CHO), which may have played a role in the prebiotic formation of the pyrimidine bases cytosine and uracil, is formed in water solutions by addition of water to cyanoacetylene (H-C C-C N), a compound that exists in interstellar space, in comets, and planetary atmospheres. A gas-phase model of the uncatalyzed addition of water to cyanoacetylene is explored by ab initio calculations at the MP2/6-311++G** level of theory. A reaction path consisting of several steps was found in these calculations, but the activation energy of the first step is relatively high, which makes it unlikely that cyanoacetaidehyde is formed in an uncatalyzed reaction. Similar calculations were also performed for the uncatalyzed reaction of water to protonated cyanoacetylene (H-C C-C N-H+), a component of the interstellar medium, forming protonated cyanoacetaldehyde (H-N CCH2CHO+), but a high activation energy was found for this reaction as well. Moreover, the corresponding addition reactions of hydrogen sulfide (H2S) to H-C C-C N, as well as to H-C C-C N-H+, have been explored with similar results.