The mechanisms for the addition reactions of phenylhalocarbenes and phenyldihalomethide carbanions with acrylonitrile (ACN) and trimethylethylene (TME) have been investigated using an ab initio BH and HLYP/6-31G (d, p) level of theory. Solvent effects on these reactions have been explored by calculations that included a polarizable continuum model (PCM) for the solvent (THF). These model calculations show that for the addition of phenylhalocarbenes, a TME species may readily undergo addition reactions with carbenes while ACN has a high-energy barrier to overcome. It was also found that phenyldihalomethide carbanions do not readily add to the electron-rich TME. The cyclopropane yields only appear to occur via addition of PhCBr to TME. However, the cyclopropanation proceeds not only via slow addition of phenylhalocarbenes to ACN but also forms through the stepwise reaction of phenyldihalomethide carbanions with ACN. Our calculation results are in good agreement with experimental observations (Moss, R.A.; Tian, J.-Z. J. Am. Chem. Soc. 2005, 127, 8960) that indicate that the cyclopropanation of phenylhalocarbenes and phenyldihalomethide carbanions with ACN are concurrent in THF.