C-4 to C-6 cycloalkynes and cycloalkylidenecarbenes have been investigated by MCSCF(4,4)/6-31G* and MP4/6-31G*//MP2/6-31G* calculations. Cyclobutyne (3) is predicted to rearrange to cyclopropylidenecarbene (4) with a barrier of 0 to 4.6 kcal/mol and a reaction enthalpy of -20 kcal/mol. Cyclobutylidenecarbene (6) is predicted to rearrange to cyclopentyne (7) with a barrier of 4 to 15 kcal/mol and a reaction enthalpy of -8 kcal/mol. Similarly, cyclopentylidenecarbene (8) is predicted to give cyclohexyne (9) with a barrier of 11 to 24 kcal/mol and a reaction enthalpy of -17 kcal/mol. The energetics are explained as a balance between the exothermicity of the carbene to alkyne reaction, ring size strain, and strain in the cycloalkyne pi bond. An unexpected conclusion is that pi bond strain does not increase in large increments with bending about the triple bond, but rather is nearly maximized for cyclopentyne and then increases slowly thereafter. In-plane pi bond strain in cyclopropyne (17), 3, 7, and 9 is estimated from isodesmic reactions to be 75.4, 73.4, 68.1, and 40.7 kcal/mol, respectively. An alternative approach based on homodesmic reactions affords total strain energies of 133, 106, 74, and 41 kcal/mol.