Carbon atom reactions with anisole and methoxybenzaldehyde demonstrate the reversible ring expansion of methoxyphenylcarbene (CH(3)O-C(6)H(4)-C-H). Trapping with HBF(4) yields the methoxytropylium ion, analogous to the well-known reactions of phenylcarbene. For instance, in the reaction of carbon atom with p-methoxybenzaldehyde, which proceeds by deoxygenation of the carbonyl group and formation of the corresponding arylcarbene, the products formed are methoxytropylium fluoroborate, p-methoxytoluene and m-methoxytoluene in yields of 69.4, 7.6, and 22.9%, respectively. Gas-phase density functional theory calculations were also carried out. The observed product yields from the uniquely generated p- and m-methoxyphenylcarbenes are in good agreement with the calculations. In the case of o-methoxyphenylcarbene, however, the calculations indicate that formation of dihydrobenzofuran is the most facile rearrangement on the ground-state singlet surface. In contrast, ring expansion is observed to be the major reaction path experimentally. The exothermicity of the deoxygenation step for carbon atom reaction with methoxybenzaldehyde ( approximately 100 kcal/mol) can allow for an excited singlet state to be formed initially. This excited singlet state of the methoxyphenylcarbene will resemble open-shell phenylnitrene, which is known to undergo ring expansion much more readily than phenylcarbene. On the basis of this analogy, we reconcile the difference in reactivity of the arylcarbene predicted by density functional theory calculations with the reactivity observed experimentally.