Ab initio potential energy surfaces have been constructed and used to carry out classical simulations of the reactions of X with HOC+ and of XH+ with CO (X=Ne, Ar, and Kr). The competition between rearrangement, X+HOC+--> OCH++X, and abstraction, X+HOC+--> XH++CO, has been examined, and found to favor abstraction in the cases where both processes are energetically allowed. The reaction of XH+ with CO is found to produce highly vibrationally excited [CHO](+) products. (C) 2000 American Institute of Physics. [S0021-9606(00)01414-8].