The cycloreversion of cyclobutane radical cation (1 c-C4H8.+)has been investigated in detail at the UB3LYP/6-31++G(d,p) level of theory. The 'rotating' mechanism of cycloreversion via the 'tightly bound' intermediate of tetramethylene radical cation (2 CH2CH2CH2CH2.+) has been revealed for the first time, It is shown that both the present 'rotating' mechanism and the previous 'shifting' one via the 'loosely bound' complex (3 C2H4 . C2H4.+) may compete intensively with each other for this reaction, leading to the low-lying isomers (4 c-CH2CH2CH-CH3.+ and 5 CH3CH2CHCH2.+) in low temperature but to the dissociation products 6 C2H4 + C2H4.+ in high temperature above 600 K. (C) 2002 Elsevier Science B.V. All rights reserved.