9-Diazofluorenes (RDAF, 1) having a series of alkyl (R) groups from Me to tBu at the 1-position were prepared and decomposed to generate the corresponding fluorenylidenes (RFL, 2) under various conditions in order to examine steric effects on the reactivities of carbenes. Thus, in cyclohexane, singlet fluorenylidenes ((1)RFLs) gave 9-cyclohexylfluorenes while triplet states ((3)RFLs) underwent H abstraction to give 9-fluorenyl (RFLH(.)), which eventually led to fluorene (4, RFLH(2)) and 9,9’bifluorenyl (5), and the ratio of the triplet products to the singlet was increased as more bulky R groups were introduced at the 1-position. These results are interpreted in terms of the steric effects on singlet reactivity, which requires formation of two bonds simultaneously. Generation of tBuFL resulted in the almost exclusive formation of intramolecular reaction products which involved not only insertion of carbene into the delta-C-H bonds of the tert-butyl group but also insertion of the 1,5-biradical, followed by neophyl-type rearrangement. The results are understood as indicating that abstraction of the delta-H by (3)tBuFL gains over the concerted intramolecular C-H insertion in (1)tBuFL. Generation of 1-RFL (R = Et, iPr, tBu) in the gas phase at high temperature gave intramolecular reaction products both in singlet and in triplet states, but the ratio of the singlet to the triplet product increased in going from Et to iPr to tBu presumably due to the increased opportunities of (1)FL to be trapped by delta-C-H bonds. Spectroscopic studies using matrix isolation techniques as well as laser flash photolysis were also carried out to gain information on the intermediates.