The mechanism for the reaction of HCCO and OH has been investigated at different high-levels of theory. The reaction was found to occur on singlet and triplet potential energy surfaces with multiple accessible paths. Rate constants predicted by variational RRKM/ME calculations show that the reaction on both surfaces occurs primarily by barrierless OH attack at both C atoms producing excited intermediates which fragment to produce predominantly CO and (HCOH)-H-1,3 with k(S) = 3.12 x 10(-8) T(-0.59)exp[-73.0/T] and k(T) = 6.29 x 10(-11) T(0.13)exp[108/T] cm(3) molecule(-1) s(-1) at T = 300-2000 K, independent of pressure at P < 76000 Torr. (C) 2013 Elsevier B.V. All rights reserved.