Reaction pathways and kinetics are analyzed as function of temperature and pressure on formation and reactions of the adduct resulting from OH addition to ethylene. Ab initio methods are used to determine thermodynamic properties of intermediate radicals, transition states (TS) and vinyl alcohol. Enthalpies of formation (Delta H(f)degrees(298) in kcal/mol) are determined for C . H2CH2OH, CH3CH2O., and CH2CHOH using CBS-q//MP2(full)6-31G(d,p) and G2 methods with isodesmic reactions, where zero point vibrational energies (ZPVE) and thermal correction to 298.15 K are incorporated. Delta H(f)degrees(298) Of TS's are determined for C . H2CH2OH (H atom shift), CH3CHO-H GO-scission to form acetaldehyde + H), CH3-CH2O (beta-scission to form formaldehyde + methyl radical) and CH2CHOH-H (beta-scission to form vinyl alcohol + H) using CBS-q//MP2(full/6-31G(d,p) and G2 methods. Entropies (S degrees(298) in cal/mol K) and heat capacities (C-p(T) 300 less than or equal to T/K less than or equal to 1500 in cal/mol K) are determined using geometric parameters and scaled vibrational frequencies obtained at the MP2(full)/6-31G(d,p) level of theory for CBS-q calculations. Geometric parameters obtained at MP2(full)/6-31G(d) level of theory and vibrational frequencies obtained at HF/6-31G(d) are used for G2 calculations. Quantum Rice-Ramsperger-Kassel (QRRK) analysis is used to calculate energy dependent rate constants, k(E), and master equation analysis is used to account for collisional stabilization. Rate constants are compared with experimentally determined product branching ratios (C . H2CH2OH stabilization: CH2O + CH3:CH3CHO + H). OH adds to ethylene to form an energized ethylene-OH adduct radical (C . H2CH2OH)*. This energized adduct can dissociate back to reactants, isomerize via hydrogen shift (E-a,E-rxn = 29.8 and 30.8 kcal/mol) to form CH3CH2O, (Delta H(f)degrees(298) = -1.7 and -3.3 kcal/mol) for CBS-q and G2 calculations respectively, or be stabilized. The CH3CH2O. isomer can undergo beta-scission reaction to either formaldehyde (CH2O) + methyl radical (CH3) (E-a,E-rxn = 13.4 and 16.0 kcal/mol) or acetaldehyde CH3CHO + H atom (E-a,E-rxn = 17.6 and 19.2 kcal/mol) for CBS-q and G2 calculations, respectively. Hydrogen atom tunneling is included by use of the Eckart formalism. Tunneling effect coefficients are 842, 93.1, and 21.1 for C . H2CH2OH --> CH3CH2O, CH3CH2O. --> C . H2CH2OH and CH3CH2O. --> CH3CHO + H at 295 K, respectively. Chemical activation and falloff are determined to be of major importance in determination of the dominant reaction paths and rate constants versus pressure and temperature in this three heavy atom system.