The unimolecular decomposition processes Of ethylene glycol. have been investigated with the QCISD(T) method with geometries, optimized at the B3LYP/6-311++G(d,P) level Among the decomposition. channels identified, the H(2)O-elimination channels have the lowest barriers, and the C-C bond dissociation is the lowest energy dissociation channel among the barrierless reactions (the direct bond cleavage reactions). The temperature and pressure dependent rate constant calculations show that the H(2)O elimination reactions are predominant at low temperature, whereas at high temperature, the direct C-C bond dissociation reaction is dominant At 1 atm, in the temperature range 500-2000 K, the calculated rate constant is expressed to be 7.63 x 10(47)T(-10.38) exp(-42262/T) for the channel CH(2)OH-CH(2)OH -> CH(2)CHOH + H(2)O, and 2.48 x 10(51)T(-11.58) exp(-43593/T) for the channel CH(2)OHCH(2)OH -> CH(3)CHO + H(2)O, whereas for the direct bond dissociation reaction CH(2)OHCH(2)OH -> CH(2)OH + CH(2)OH the rate constant expression is 1.04 x 10(71)T(-16.16) exp (-52414/T).