We report the first measured activation parameters for the additions of CCl2 and CClF to simple alkenes and demonstrate the existence of enthalpic barriers for CCl2 additions to cyclohexene and 1-hexene. With these two alkenes, additions of PhCCl are "dominated" by entropic contributions to deltaG++ and additions of CCl2 display comparable contributions of deltaH++ and deltaS++, while CClF additions feature dominant enthalpic contributions to deltaG++. Entropic factors, however, control the additions of all three carbenes to the highly reactive alkene, tetramethylethylene. Parallel computational studies are also reported.