All possible pi bonds formed between the elements C, N, 0, Si, P, and S are considered. The pi bond strengths are estimated by the cis-trans rotation barriers (where possible) and by hydrogenation energies. The ability of these elements to form strong pibonds is in the order 0 > N "" C » S > P > Si. In addition, computed bond lengths and vibrational stretching frequencies are reported for both the singly and doubly bound compounds. The structure of the lowest triplet state of each double-bonded compound is given, along with the singlet-triplet splitting.