The photochemistry of butatriene (2), matrix-isolated in argon at 10 K, was investigated. UV irradiation (248 nm) produces vinylacetylene (1) and the dimer of acetylene and in addition methylenecyclopropene (3) as a major product. The photochemical 2 --> 3 rearrangement is reversible, and on visible light irradiation (lambda > 420 nm) 3 is transformed back to 2, The formation of 3 from 2 requires a [1,2]-H migration to allenylcarbene (8), According to MP2 and DFT calculations, 8 is a minimum on both the triplet (T) and the singlet (S) potential energy surface (PES). The comparison of measured and calculated infrared spectra suggests that two forms of 8 (either syn and anti form of S-8 or a mixture of S-8 and T-8) can be trapped at low temperatures in the matrix. The rearrangement 2 --> 3 proceeds in a stepwise manner with S-8 as intermediate on the S-PES (calculated heat of formation at 298 K, Delta H degrees(f)(298) = 131.7 kcal/mol) and T-8 on the T-PES (Delta H degrees(f)(298)= 129.0 kcal/mol).