irradiation (lambda > 300 nm) of 1-naphthyldiazomethane (11) or 1-naphthyldiazirine (12) isolated in an argon matrix at 10 K produces 4,5-benzobicyclo[4.1.0]hepta-2,4,6-triene (2) and a small amount of triplet 1-naphthylcarbene (1). Cyclopropene 2 is characterized by infrared and UV/visible spectroscopy while triplet 1-naphthylcarbene (1) is characterized by infrared, UV/visible, and ESR spectroscopy. Experimental and computed (B3LYP/6-31G*) IR spectra display good agreement. Carbene 1 and cyclopropene 2 are thermally stable in the matrix at 10 K, although they may be interconverted photochemically. Further irradiation of the matrix under a number of conditions (lambda > 300, lambda = 342 +/- 10, lambda > 497, or lambda = 544 +/-10 nm) causes disappearance of carbene 1 and exclusive growth of cyclopropene 2. Shorter wavelength photolysis (lambda = 290 +/- 10 nm) triggers reversion of 2 back to 1. Conversion of either compound to 4,5-benzocyclohepta-1,2,4,6-tetraene (6), 2-naphthylcarbene (8), or 2,3-benzobicyclo-[4.1.0]hepta-2, 4,6-triene (9) was not observed under any of our experimental conditions. Matrix isolated cyclobuta[de]naphthalene (4) disappears slowly upon prolonged UV irradiation (lambda > 200 nm), affording a trace of triplet 1-naphthylcarbene (1).