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Summary. The absolute rate constants determined for the additions of FCPh, ClCPh, and BrCPh to Me2C=CMez, MepC=CHMe, trans-MeCH=CHEt, and n-BuCH=CH2 appear to follow a reactivity/selectivity pattern of the "normal " (inverse) type. Much has been learned about the reactivity and selectivity of carbenes from studies of the relative rates of their cycloadditions to various a1kenes.l Now, however, nanosecond flash photolytic techniques permit the direct measurement of absolute rate constants and activation parameters for carbene cycloadditions. We have previously applied the new methods to the addition reactions of ClCPh with alkenes,2 and have obtained Hammett correla-tions for the additions of a series of E-X-PhCC1.3 Here, we report kinetic data which con-stitute the first examples, obtained by direct measurements, of the dependence of carbenic reactivity on substituent variation at the carbenic carbon.- The new results are important because they provide clear evidence for a "normal " reactivity/selectivity correlation in carbene/alkene addition reactions. Although such a relationship has long been assumed,l its general occurrence is controversial4 and it was not observable in our Hammett study.3 Halophenylcarbenes &- & were generated by steady state photolysis (A>300 nm) o