Highly correlated ab initio molecular orbital calculations have been used to map out the potential energy surface of the C1 + allene reaction in the gas phase. Eight transition state structures governing the mechanism of the title reaction were computed at seven different levels of theory. The results show that UMP2 calculations are adversely affected by contamination from higher spin states. QCISD(T) calculations illustrate the importance of correlation including triple electron excitations in the quadratic configuration wave function for obtaining an accurate description of the potential energy surface. Results computed at the QCISD(T)/6-31+G(d,p)// QCISD/6-31+G(d,p) level indicate that chlorine atom addition at the center and end carbons of allene are barrierless processes and that the chemically activated C3H4CI complex may isomerize through chlorine atom transfer but not through hydrogen atom transfer. The metathesis transition state corresponding to hydrogen abstraction by chlorine lies 15 kJ(.)mol(-1) above reactants. The isomerization reaction path between the 3-chloroI-propen-2-yl and 2-chloroallyl radical adducts lies below the initial reactants, permitting the incipient C3H4Cl ensembles to establish equilibrium. Thus, the dominant addition product of C1 + allene is the 2-chloroallyl radical.