Ketenyl radical (HCCO) is an important hydrocarbon combustion intermediate. The mechanisms and kinetics for the reaction of HCCO (X(2)A '') with H(S-2) occurring on both singlet and triplet surfaces have been studied by a combination of ab initio calculations and rate constant predictions at the CCSD(T)/6-311++ G(3df,2p)//CCSD/6-311++G(d,p) level of theory. The kinetics and product branching ratios have been investigated in the temperature range of 297-3000 K by variational transition state and Rice-Ramsperger-Kassel-Marcus (RRKM) theories for the production of CH2(a(1)A(1)) + CO(X-1 Sigma(+)) and CH2((XB1)-B-3) + CO(X-1 Sigma(+)). Our prediction for the primary product CH2(a(1)A(1)) + CO(X-1 Sigma(+)) formation is in good agreement with earlier experimental results. The pressure independent rate constant for this channel can be expressed by k(1) (T) = 8.62 x 10(-11)T(0.16)exp(-20/T) cm(3) molecule(-1) s(-1). For the production of CH2((XB1)-B-3) + CO(X-1 Sigma(+)), the rate constant k(2) can be represented as k(2)(T) = 7.63 x 10(-16)T(1.56)exp(-386/T) cm(3) molecule(-1) s(-1). The predicted product branching ratios for the reaction are in close agreement with experimental data as well. We also predicted the heat of formation at 0 K for (HCCO)-H-2, (CCO)-C-3, and (CCO)-C-1 by CCSD(T)/6-311++ G(3df,2p), CBS-QB3, and G2M; the values are in good agreement among one another. Specifically, the CCSD(T) values are:Delta H-f degrees(HCCO, X2A '') = 42.52 +/- 0.70; Delta H-f degrees(CCO, (XSg)-S-3) = 91.50 +/- 0.54; and Delta H-f degrees(CCO, a(1)Delta) = 110.22 +/- 0.54 kcal/mol.